if the strokeSize is < 1 in antialaising, SkScan::AntiFrameRect may draw wrongly (potentially twice on the same pixels).

SkScan::AntiFrameRect wants to draw in 3 stages:
1. skim the outer rect of the frame, computing partial coverage
2. fill in the sides/top/bottom with solid depending on how thick the stroke is
3. skim the inner rect of the frame, computing partial coverage

For thin rects, #2 is correctly skipped, but we still do #1 AND #3. We need to detect when the stroke lies wholly within 1 pixel, and just skim it once, merging the coverage of the inner and outer rect.