Use official headers to define GL_ARB_texture_rectangle.

Use official headers to define GL_ARB_texture_rectangle.

Fixing https://github.com/KhronosGroup/OpenGL-Registry/pull/149 and
https://github.com/KhronosGroup/OpenGL-Registry/pull/183 enabled
including desktop OpenGL headers without conflicts, so we no longer
have to define enums for ARB extensions ourselves.

Change-Id: I4f2e2d3b79a6f5cacf53f2a8ace78b6d124cb5d1
Reviewed-on: https://swiftshader-review.googlesource.com/19029
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>