st-api: Clean-up OpenGL profile handling

st-api: Clean-up OpenGL profile handling

There seems to have been two different ways to communicate the
profile.  There were flags and there were profiles.  I've opted to
remove the profile flags and use ST_PROFILE_DEFAULT (compatibility
profile) and ST_PROFILE_OPENGL_CORE (core profile) consistently
instead.

Also change the values of the ST_CONTEXT_FLAG_DEBUG and
ST_CONTEXT_FLAG_FORWARD_COMPATIBLE flags to match the WGL and GLX
values.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Chia-I Wu <olv@lunarg.com>