gl/msaa:  reduce clip complexity when converting clip to traps

We draw clips during paint and mask operations in msaa compositor.
clip can be larger than source or mask pattern.  We intersect clip
with source and mask pattern to reduce clip complexity and such reduce
time to tessellate the clip when operator is CAIRO_OPERATOR_OVER
shadow:  do not cache shadow if paint or mask is not bounded
shadow: add shadow caching support for image backend