glapi: add ARB_texture_multisample

glapi: add ARB_texture_multisample

Adds new enums, dispatch machinery, and stubs for the 4 new entrypoints.

V2: - Drop placeholder
    - Align enum values
    - Remove explicit exec=mesa; it *is* the dispatch flavor we want,
      but it's also the default. I misunderstood how this worked before;
      after actually reading the generator it makes good sense.

V3: - Squash in stubs for new entrypoints, and dispatch_sanity tweaks,
      so we don't get build breakage between those patches.

V4: - Fix various remaining whitespace issues

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
[1/3 V2] Reviewed-by: Matt Turner <mattst88@gmail.com>
[V3] Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>