shape-outside image drops an above-threshold run that starts in the last column

https://bugs.webkit.org/show_bug.cgi?id=326241
rdar://189116534

Reviewed by Simon Fraser.

This patch aligns WebKit with Gecko / Firefox and Blink / Chromium.

LayoutShape::createRasterShapeImpl() scans each row of the shape image and
builds intervals from runs of pixels whose alpha is above the threshold. A
run was only emitted from the else-if branch, which also handled the last
column. When the run started on the last column, the first branch opened
it and the else-if never ran, so the loop ended without emitting the run.
As a result, a 1px wide shape image had an empty shape, and a single
above-threshold pixel in the last column was ignored, in both horizontal
and vertical writing modes.

Fix this by emitting any run that is still open after the row loop ends,
and remove the last-column special case from the else-if.

Test: imported/w3c/web-platform-tests/css/css-shapes/shape-outside/shape-image/gradients/shape-outside-linear-gradient-last-column.html

* LayoutTests/imported/w3c/web-platform-tests/css/css-shapes/shape-outside/shape-image/gradients/shape-outside-linear-gradient-last-column-expected.txt: Added.
* LayoutTests/imported/w3c/web-platform-tests/css/css-shapes/shape-outside/shape-image/gradients/shape-outside-linear-gradient-last-column.html: Added.
* Source/WebCore/rendering/shapes/LayoutShape.cpp:
(WebCore::LayoutShape::createRasterShapeImpl):

Canonical link: https://commits.webkit.org/322756@main