i965: Change fragment input related bitfields to 64-bit.

This patch updates the bitfields brw_context::wm.input_size_masks,
tracker::size_masks, and brw_wm_prog_key::proj_attrib_mask, all of
which are indexed by gl_frag_attrib, from 32-bit to 64-bit.

This paves the way for supporting geometry shaders, and for merging
the gl_frag_attrib and gl_vert_result enums.  The combination of these
two will require at least 55 bits in the bitfields.
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.
mtypes.h: Modify gl_vert_result to refer to new gl_varying_slot enum.
Replace gl_vert_result enum with gl_varying_slot.
mtypes.h: Modify gl_geom_attrib to refer to new gl_varying_slot enum.
Replace gl_geom_attrib enum with gl_varying_slot.
mtypes.h: Modify gl_geom_result to refer to new gl_varying_slot enum.
Replace gl_geom_result enum with gl_varying_slot.
mtypes.h: Modify gl_frag_attrib to refer to new gl_varying_slot enum.
Get rid of _mesa_vert_result_to_frag_attrib()
Get rid of _mesa_frag_attrib_to_vert_result()
Replace gl_frag_attrib enum with gl_varying_slot.