Merged recent bug fixes from the USD implementation of the
GL Api Loader. This fixes the OpenGL system framework path on
macOS and also improves GL version and GL extension processing.
At build time, the preprocessor symbols: OSD_USES_GLEW or
OSD_USES_INTERNAL_GLAPILOADER determine the GL API loader
that will be used by the OSD library.
This extends the internal glLoader library to have a fully
internally implemented GL API loader so that the OpenSubdiv
libraries can be used independent of any dependencies on an
external GL API loader like GLEW.
This introduces an internal glLoader library which allows
most of the implementation to be agnostic about the
implementation of the GL loading library. Specifically,
this removes references to the GLEW headers and libraries
from the rest of the source code and build system.
