mesa: implement sample mask

mesa: implement sample mask

V2: - fix multiline comment style
    - stop using ASSERT_OUTSIDE_BEGIN_END_AND_FLUSH since that
      doesn't exist anymore.

V3: - check for the extension being enabled
    - tidier flagging of _NEW_MULTISAMPLE
    - fix weird indentation in get.c

V4: - move flush later in SampleMaski()

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>