mesa: allow MESA_GL_VERSION_OVERRIDE to override the API type

mesa: allow MESA_GL_VERSION_OVERRIDE to override the API type

Change the format to MAJOR.MINOR[FC]
For example: 2.1, 3.0FC, 3.1

The FC suffix indicates a forward compatible context, and
is only valid for versions >= 3.0.

Examples:
2.1:   GL Legacy/Compatibility context
3.0:   GL Legacy/Compatibility context
3.0FC: GL Core Profile context + Forward Compatible
3.1:   GL Core Profile context
3.1FC: GL Core Profile context + Forward Compatible

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>