Do be sure to rewind in bitmap test, to fix broken GIF loader
Merge branch 'fix-missing-rewinding' into develop

Conflicts:
	converters/stb_image.h
Merge branch 'develop' into stb_image
Add a test GIF image
Merge branch 'stb_image' into release-0.21
Update version
Merge branch 'release-0.21' into release
Merge branch 'release'