Add ability to save screenshot

[slimbuck]

Fixes: #238

This PR:

• adds a menu option "Save Screenshot" which will download a PNG version of the viewport.
• use lodpng for compression since inbuilt browser compression can destroy images with alpha
• add automatic translations for the other languages:
  • french version is confirmed
  • if you have a chance, please check the German @simonbethke
  • anyone else for the rest :)

Since the backbuffer is generated with premultiplied alpha, we must apply a background color before compression.

In future we should consider adding more screenshot options:

• save without background (i.e. semitrans) or specify a background color
• optionally remove the debug visuals (grid, overlay, bounding box) so user doesn't have to do this manually
• support video output of the currently loaded camera path