[skottie] Fix zero-overlap selector handling

The current implementation applies constant coverage (outside selector
range) based on computed integral edges.

But the integral range is clamped to the valid index domain and its
extremes are always assumed to have partial coverage - so we never get
to constant-blit the full buffer when the interval is outside, which
can yield incorrect coverage for the first/last fragments.

Update the constant coverage logic to operate in full domain coordinates.

Change-Id: I23902674fe5e822081fb8262167511df1cc3463e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/219206
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed