Blender Git Loki

New build option WITH_LEGACY_OPENGL

This introduces a new CMake option - WITH_LEGACY_OPENGL. Without this option
things may not draw perfectly, however, we should soon be able to build with
OpenGL core profile.

The matrix-related api calls are (still) not handled here (glTranslate, ...).

There seems to be no consensus on whether to make this build option the
default. We can talk about this later. For now two things are the
priority:

(1) To get rid of deprecated calls when WITH_LEGACY_OPENGL is ON
(2) To make core profile work for Mesa/Mac when WITH_LEGACY_OPENGL is OFF

Reviewers: merwin, sergey, campbellbarton

Differential Revision: https://developer.blender.org/D2603

Many thanks for Sergey Sharybin for the help.