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Cull simple non-empty canvas text drawing outside of the clip
https://bugs.webkit.org/show_bug.cgi?id=306394
rdar://172308695

Reviewed by NOBODY (OOPS!).

In simple cases where no-frills text would be drawn on top of a canvas,
it's easy and fast to check if the text does NOT intersect with the
current clip, in which case the whole operation can safely be skipped.

As an exception, if the textRect is computed to be empty (zero width or
height), it is assumed to be of unknown but actually-non-empty size, so
the optimization is just skipped. This may be used by some websites to
obscure privacy-sensitive texts.

* LayoutTests/fast/canvas/fillText-edge-clip-expected.html: Added.
* LayoutTests/fast/canvas/fillText-edge-clip-shadow-expected.html: Added.
* LayoutTests/fast/canvas/fillText-edge-clip-shadow.html: Added.
* LayoutTests/fast/canvas/fillText-edge-clip.html: Added.
* LayoutTests/fast/canvas/fillText-edge-expected.html: Added.
* LayoutTests/fast/canvas/fillText-edge-shadow-expected.html: Added.
* LayoutTests/fast/canvas/fillText-edge-shadow.html: Added.
* LayoutTests/fast/canvas/fillText-edge.html: Added.
* LayoutTests/fast/canvas/fillText-empty-clip-expected.html: Added.
* LayoutTests/fast/canvas/fillText-empty-clip.html: Added.
* LayoutTests/platform/win/TestExpectations:
* Source/WebCore/html/canvas/CanvasRenderingContext2DBase.cpp:
(WebCore::CanvasRenderingContext2DBase::drawTextUnchecked):

a37ac2d

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