If we have multiple damage rects we would sync (if we would sync) after
every hostx_paint_rect. For shm images you'd rather push all the
ShmPutImage requests and wait after the last one.

      before                  after   Operation
------------   --------------------   -------------------------
 232000000.0    240000000.0 (1.034)   Dot
  41500000.0     41400000.0 (0.998)   1x1 rectangle
  11400000.0     11400000.0 (1.000)   10x10 rectangle
    553000.0       553000.0 (1.000)   100x100 rectangle
     37300.0        38500.0 (1.032)   500x500 rectangle
    831000.0      1140000.0 (1.372)   PutImage 10x10 square
     65200.0       134000.0 (2.055)   PutImage 100x100 square
      3410.0         3500.0 (1.026)   PutImage 500x500 square
    810000.0      1150000.0 (1.420)   ShmPutImage 10x10 square
    346000.0       364000.0 (1.052)   ShmPutImage 100x100 square
     22400.0        22800.0 (1.018)   ShmPutImage 500x500 square

Reviewed-by: Emma Anholt <emma@anholt.net>