This reuses the Cycles regression test code to also work for OpenGL UI drawing.
We launch Blender with a bunch of .blend files, take a screenshot and compare
it with a reference screenshot, and generate a HMTL report showing the failed
tests and their differences.
For Cycles we keep small reference renders to compare to in svn, but for OpenGL
developers currently have to generate the references manually. How to use:
* WITH_OPENGL_DRAW_TESTS=ON in CMake
* BLENDER_TEST_UPDATE=1 ctest -R opengl_draw
* .. make code changes ..
* ctest -R opengl_draw
* open build_dir/tests/opengl_draw/report.html
Differential Revision: https://developer.blender.org/D3064
