In some situations, the items within a filter stacking context will
have clips applied. In this case, we minimize the allocated size
of the intermediate surface, to avoid drawing redundant information
that won't be seen.

However, if the stacking context itself does not have the same clip
applied, then we need to ensure that when we draw the blend primitive,
we don't sample outside the smaller region of the intermediate
surface that was rendered.

Ideally, the stacking context should have a clip applied to it so
that we don't waste time drawing transparent pixels. Adding a clip
to the stacking context here would provide an optimization win in
many cases. However, even without one present, we should still
ensure correct rendering, thus this fix is required.

Also fix a case where brush primitives with segments were not setting
the correct bounding rect for a render task. This wasn't causing a
problem here, but may have caused other issues.