common sw_engine: optimize single rectangle ClipPath.

common sw_engine: optimize single rectangle ClipPath.

If ClipPath is a singular rectangle,
we don't need to apply this to all children nodes to adjust rle span regions.

Rather than its regular sequence,
we can adjust render region as merging viewport that is introduced internally,

All in all,
If a Paint has a single ClipPath that is Rectangle,
it sets viewport with Rectangle area that viewport is applied to
raster engine to cut off the rendering boundary.

In the normal case it brings trivial effects.
but when use SVGs which has a viewbox, it could increase the performance
up to 10% (profiled with 200 svgs rendering at the same time)

Note that, this won't be applied if the Paint has affine or rotation transform.

@Issues: 294