mtypes.h: Add new gl_varying_slot enum, and bitfield defines.

Future patches will make use of the enum.  It will eventually take the
place of the existing enums gl_vert_result, gl_geom_attrib,
gl_geom_result, and gl_frag_attrib, all of which represent essentially
the same information but using inconsistent values.
WIP: Remap VERT_RESULT to new enum