Restore SVG filter optimization that was lost in the deferral switch.

The legacy filter path does some optimizations to filter bounds in the
case where the filter is known not to affect transparent black, or can
be restricted to a subset of its input rects. Although we should
probably move these optimizations to Skia in the medium term, for now
this change simply invokes them from the deferred path as well.

This caused some minor pixel diffs at the clip boundaries. 
Since many of these same tests were also affected by an 
unnecessary snapping to integer pixels (that I've been 
meaning to remove -- it makes the results worse, and is 
wrong in the pinch-zoom case anyway), I've removed that
here as well.

BUG=408031

Review URL: https://codereview.chromium.org/498073003

git-svn-id: svn://svn.chromium.org/blink/trunk@181007 bbb929c8-8fbe-4397-9dbb-9b2b20218538