Enter RectangleMode when the clip region is a rectangle

ClipArea's region mode implementation store the region's boundary in
a rectangle, and if the region is that rectangle, set its region to
empty.  This is a bug, since as long as the ClipArea is in region
mode it treats its region as determinitive. When the next clipping
operation comes in, it is tested against the empty region instead of
against the valid rect.

Discovered via Skia GM testing.

Change-Id: I3bbc4e81c8d68636061f830156e017e9b5c7e587