Old code had a mix of framebuffer error codes from OpenGL ES, EXT_framebuffer_object, and desktop GL. We use desktop GL (or ARB_framebuffer_object which acts just like GL 3.x) so I made it compatible with that.
Changed messages to the actual GL_FRAMEBUFFER_XXX symbols. These are less friendly and more accurate. Can easily look up what an error means, unfiltered by what a Blender dev thinks it means.
Kept ES error codes around in case we support that one day. Just flip the #if or use a compile-time option.
ARB_framebuffer_object replaces several related EXT extensions. The ARB
version pulls GL 3 FBO features into GL 2.1, useful for Mac platform.
Its functions and enums have no ARB suffix so transition to modern GL
will be seamless!
Extension is checked at startup, so is guaranteed to be true at runtime.
Part of T49012
