glapi: add ARB_texture_rg

glapi: add ARB_texture_rg

Noticed it was missing based on the lack of a descriptive enum
name from this bug's error message:

https://bugs.freedesktop.org/show_bug.cgi?id=44039

This moves two enums out of GL3x.xml. Though since this and
GL_ARB_texture_compression_rgtc are both strict subsets of GL3,
both extensions should have had all their enums in that file
to begin with, not just two of them.

Reviewed-by: Brian Paul <brianp@vmware.com>