Fix SkClipStack bug with inverse-filled difference elements

Fix SkClipStack bug with inverse-filled difference elements

Previously, SkClipStack would call "setEmpty" on itself when an
inverse-filled difference element made the stack empty. This was
a problem because setEmpty would forget the element had an inverse
fill, yet leave the op as "difference". This change modifies it to
manually update the clip bounds and set the gen-ID to kEmptyGenID,
rather than calling setEmpty.

BUG=skia:
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Review-Url: https://codereview.chromium.org/2175493002