mesa: Track the OpenGL API we're implementing in the context

mesa: Track the OpenGL API we're implementing in the context

This introduces a new way to create or initialize a context:

  _mesa_create_context_for_api and
  _mesa_initialize_context_for_api

which in addition to the current arguments take an api enum to indicate
which OpenGL API the context should implement.  At this point the
API field in GLcontext isn't used anywhere, but later commits will
key certain functionality off of it.

The _mesa_create_context and _mesa_initialize_context functions are
kept in place as wrappers around the *_for_api versions, passing in
API_OPENGL to get the same behavior as before.