OpenGL: fix placeholder sampler in vertex stage. #4236

Job	Run time
Go Build and Test	45s
CMake-windows-latest-Debug-msvc	13m 51s
CMake-windows-latest-Release-msvc	16m 51s
CMake-ubuntu-latest-Debug-gcc	1h 2m 33s
CMake-ubuntu-latest-Release-gcc	1h 6m 6s
CMake-macos-latest-Debug-clang	19m 11s
CMake-macos-latest-Release-clang	21m 10s
CMake-macos-13-Debug-clang	56m 14s
CMake-macos-13-Release-clang	52m 25s
	5h 9m 6s

Provide feedback