Transform feedback

[mosra]

Read, learn, undestand, design and implement support for

• EXT_transform_feedback (GL 3.0)
• ARB_transform_feedback2 (GL 4.0)
• ARB_transform_feedback3 (GL 4.0)
• ARB_transform_feedback_instanced (GL 4.2)
• ARB_transform_feedback_overflow_query (ARB extension)
• AMD_transform_feedback3_lines_triangles

and equivalent subset found in ES 3.0 / 3.1.

Currently the main use case is GPU-powered particle systems and physics on hardware that doesn't support generic compute shaders.

[mosra]

Full support for EXT_transform_feedback, transform feedback objects from ARB_transform_feedback2, advanced interleaving from ARB_transform_feedback3 and equivalent ES3 functionality is done in 6f355c5, see src/Magnum/Test/TransformFeedbackGLTest.cpp for some sample code. I'll add more documentation, some example and remaining features later.

I decided to not support object-less transform feedback, thus you need at least ARB_transform_feedback2 or GL 4.0. Hope that's not an issue.

[mosra]

Support for XFB streams and indexed queries from ARB_transform_feedback3 is done in f275143, support for line/triangle streams from AMD_transform_feedback3_lines_triangles is done in 9373a2e, support for (classic, stream, instanced, stream instanced) transform feedback draw from ARB_transform_feedback2 and ARB_transform_feedback3 done in a60dc74 and ARB_transform_feedback_overflow_query support added in 9642869.

There are some minor state queries that are not implemented, but other than that I consider the transform feedback support complete now.

Cc: @Squareys