This test creates a similar surface using cairo_push_group () and
assumes its size will be w x h. This is not true when testing similar
surfaces with "cairo-test-suite -a", because they are bigger than the
actual ROI, which corresponds to the rectangle (0,0) - (w,h).
Clipping to the ROI guarantees that the surface created by
cairo_push_group () always has the same size.
Fixes surface-pattern-scale-down-extend.
Franz Schmid reported a regression in 1.9.6 when downscaling
and using EXTEND_REPEAT for an image surface pattern. This
patch adds such tests for every extend mode.