There's a fairly complicated interaction between an SVG gradient's paint
transformation and the gradient coordinate transformation required to
correctly draw gradient fills. This was especially noticeable when
scaling down an SVG, resulting in broken gradient coordinates and
graphical glitches.
This changes the objectBoundingBox units to immediately map to the
bounding box's coordinate system, so we can unify the gradient paint
transformation logic and make it a lot simpler. We only need to undo the
bounding box offset and apply the paint transformation to fix a lot of
gradient fill bugs.
With the newly supported fuzzy matching in our test-web runner, we can
now define the expected maximum color channel and pixel count errors per
failing test and set a baseline they should not exceed.
The figures I added to these tests all come from my macOS M4 machine.
Most discrepancies seem to come from color calculations being slightly
off.
In commit 1b82cb43c2 I accidentally
removed the paint transformation altogether. The result was that
zoomed-in SVGs, or SVG elements with a transformation applied could have
their gradient coordinates misplaced significantly.
This was also exposed in the `svg-text-effects` test by way of a slight
visual difference. Add a new test that very clearly exposes the fixed
issue by rotating the gradient coordinates by 45 degrees.
