Move CanvasFrontend off of SkConservativeClip

Implements clip bounds tracking for intersect and difference
directly. Also adds implementations for quickReject. Switches
the separate clip/transform stack entries to use deferred
saves so that they are only allocated when modified.

Test: existing tests still pass
Change-Id: Ic2d5b87d6f686e95bc6579750ecd63cde4a13143