Converts GrClip to an abstract base class and adds a "GrFixedClip"
implementation. GrFixedClip denotes a clip implemented with fixed-
function hardware. GrFixedClip allows us to remove the stateful
"fClipMode" member from GrClipMaskManager, and in the future will
be able to nicely encapsulate window rectangles.
After this change GrClipMaskManager is just a wrapper around
GrDrawTarget. We may want to consider removing it altogether.
BUG=skia:
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Review-Url: https://codereview.chromium.org/1971343002
846c051a48