Same trick as in fb: just do a normal GetImage and deal with the
planemask on the CPU if you have to. Since the software fallback hit for
glamor is pretty brutal, this is a much more impressive win for glamor
than it was for fb:

  11100.0  87700.0 (7.901) (copy 0xaaaaaaaa) ShmGetImage 10x10 square
   9840.0  47800.0 (4.858) (copy 0xaaaaaaaa) ShmGetImage 100x100 square
   1550.0   4240.0 (2.735) (copy 0xaaaaaaaa) ShmGetImage 500x500 square
   9450.0  78900.0 (8.349) (0xaaaaaaaa) GetImage 10x10 square
   6910.0  30900.0 (4.472) (0xaaaaaaaa) GetImage 100x100 square
    431.0   2020.0 (4.687) (0xaaaaaaaa) GetImage 500x500 square

Measured with Xephyr -glamor on Skylake GT3e.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>