imgui: make backends available as optional components

[valgur]

Summary

Changes to recipe: imgui/[*]

Motivation

There are two major issues with the current approach of building only the core imgui.cpp and no backends or other optional components:

• Difficult to use. The consuming projects will need to inject the <package_folder>/res/... source files into the build system in order to use the optional components. This is manageable but very inconvenient compared to the alternative of linking against a suitable component target.
• Tends to break shared builds. Imgui uses a IMGUI_API export macro for shared builds and visibility handling. The users need to take care that the core library symbols provided by the imgui recipe and the optional components built by the consuming projects have the correct IMGUI_API corresponding the static/shared build of either project. This is tricky to manage and easy to mess up.

In addition to that, the current recipe does not provide a way to modify imconfig.h. These values cannot be changed after the core library has already been built.

Another minor benefit is that the Conan and system dependencies for the optional components are handled by package_info() in the imgui recipe and don't need to be duplicated in the consuming projects.

Details

The PR aims to be backwards-compatible. imgui::imgui CMake target still links against the core library only. All backends can be disabled altogether by setting build_backends=False for the old behavior.

These changes are not without their downsides:

• Significantly more complex recipe. Imgui famously not providing any kind of build system outside of examples does not help in this regard.
• Potential for option conflicts. The CCI recipe will likely need to build with most if not all backends enabled (or at the very least OpenGL and Vulkan ones) to avoid conflicts on CCI itself.
• Recipes listing imgui::imgui as a cpp_info.requires either explicitly or implicitly no longer link against just the core lib but against all the optional components as well.

Given these downsides, I was initially hesitant to open this PR and shelved it for a while, but after hitting a wall in the Ogre PR (#21073) when trying to unvendor ImGui there, this PR started to look a lot more reasonable. That is, Ogre expects the optional Freetype component to be available and building it as part of the Ogre recipe hits the shared build issues listed above.

Test build with nearly all options enabled (only DirectX 11 and DirectX 12 disabled): #25325 (comment)

---

• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

[github-actions]

Hooks produced the following warnings for commit 9f6b90e

imgui/1.86@#f71b2c36f40b6b38a05c882e11fe20bf

post_package(): WARN: [APPLE RELOCATABLE SHARED LIBS (KB-H077)] install_name dir of these shared libs is not @rpath: libimgui-osx.dylib, libimgui-metal.dylib

imgui/1.85@#5b7ab2f3ec1b8f0fe42e0d230d1d99a7

post_package(): WARN: [APPLE RELOCATABLE SHARED LIBS (KB-H077)] install_name dir of these shared libs is not @rpath: libimgui-osx.dylib, libimgui-metal.dylib

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 26 (2b349e6adb18b2e97f86dfd761c468feb8c48a82):

• imgui/1.91.2:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.9:
  Built 18 packages out of 22 (All logs)

• imgui/1.91.0:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.9-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.5-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.91.2-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.5:
  Built 18 packages out of 22 (All logs)

• imgui/1.89.9-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.88:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.6:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.6-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.87:
  Built 18 packages out of 22 (All logs)

• imgui/1.89.9:
  Built 18 packages out of 22 (All logs)

• imgui/1.86:
  All packages built successfully! (All logs)

• imgui/1.90.7-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.8-docking:
  Built 18 packages out of 22 (All logs)

• imgui/1.85:
  All packages built successfully! (All logs)

• imgui/1.90.7:
  Built 18 packages out of 22 (All logs)

• imgui/1.90.8:
  Built 18 packages out of 22 (All logs)

• imgui/1.91.0-docking:
  Built 18 packages out of 22 (All logs)

---

Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 26 (2b349e6adb18b2e97f86dfd761c468feb8c48a82):

• imgui/1.91.2:
  Built 6 packages out of 10 (All logs)

• imgui/1.91.2-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.5:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.5-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.91.0:
  Built 6 packages out of 10 (All logs)

• imgui/1.91.0-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.9-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.6:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.8-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.9:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.8:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.7-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.6-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.88:
  Built 6 packages out of 10 (All logs)

• imgui/1.90.7:
  Built 6 packages out of 10 (All logs)

• imgui/1.89.9:
  Built 6 packages out of 10 (All logs)

• imgui/1.86:
  All packages built successfully! (All logs)

• imgui/1.89.9-docking:
  Built 6 packages out of 10 (All logs)

• imgui/1.87:
  Built 6 packages out of 10 (All logs)

• imgui/1.85:
  All packages built successfully! (All logs)

[github-actions]

Hooks produced the following warnings for commit 2b349e6

imgui/1.86@#6c07b2eebf35b7365ace7efe78b7e7de

post_package(): WARN: [APPLE RELOCATABLE SHARED LIBS (KB-H077)] install_name dir of these shared libs is not @rpath: libimgui-osx.dylib, libimgui-metal.dylib

imgui/1.85@#d6f43ac34007b753a47994a8d1303440

post_package(): WARN: [APPLE RELOCATABLE SHARED LIBS (KB-H077)] install_name dir of these shared libs is not @rpath: libimgui-osx.dylib, libimgui-metal.dylib

[valgur]

The recipe is ready for review.