Most rounded rect clips in real content are (a) axis aligned and
(b) have uniform radii.
When these conditions are met, we can run a fast path for clip
mask generation that uses significantly fewer ALU shader ops.
This is not typically a bottleneck on desktop GPUs, but can have
a large performance impact on mobile GPUs (and perhaps low end
integrated GPUs).
The Mali shader analyzer reports the slow path for the rounded
rect clip shader to be 94 cycles per fragment, while the fast
path is 10 cycles.
Differential Revision: https://phabricator.services.mozilla.com/D23817
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