This adds basic support for clip regions. It is currently disabled at compile
time. Enabling clip regions will require setting up a stencil buffer.
Change-Id: I638616a972276e38737f8ac0633692c3845eaa74
The Snapshot class is getting complicated enough that its implementation
should now live in a separate .cpp file. This will become particularly
useful when support for clip regions and paths will be added later on.
Change-Id: I050fac5683a9f7a0ff2f7a6beec3dd28aa5eb0d8
