fix gaussian blur getting clipped with negative scale

[walley892]

Fixes #183884

GaussianBlurFilterContents::GetFilterSourceCoverage decides how much to expand the bounding rect to account for the applied blur. If given a matrix with a negative scale, GetFilterSourceCoverage would shrink the bounding rect instead of expanding it - giving the appearance of the "offset" described in #183884

Added a unittest and golden test to verify the change.

Repro app on MacOS with --enable-impeller at HEAD:

Repro app with this change with --enable-impeller:

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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[gemini-code-assist]

Code Review

This pull request fixes a bug in Gaussian blur coverage calculation with negative scaling. The change in GaussianBlurFilterContents::GetFilterSourceCoverage now uses effect_transform.TransformDirection(blur_radius).Abs() to ensure the coverage rectangle is always expanded. The PR adds a new unit test, FilterSourceCoverageNegativeScale, to verify this logic and a new golden test, GaussianBlurFlipped, for visual regression testing. My review includes one suggestion to improve the new unit test.

[gaaclarke]

lgtm, thanks!

[gaaclarke]

no action required: TransformDirection? That's a new one to me.

[walley892]

Basis() just creates a new matrix where the translation is zeroed out

flutter/engine/src/flutter/impeller/geometry/matrix.h

Line 239 in 0d76183

constexpr Matrix Basis() const {

.

Instead of creating a new matrix, TransformDirection just uses the existing matrix and ignores the translation.

[gaaclarke]

Yea, I see they are reducing operations. I'm a bit skeptical that the optimization is worth introducing the new concept though. The new matrix is on the stack and with vectorization the extra ops might have been free.

[walley892]

Aye aye, I changed back to the manual matrix creation + multiplication.

[gaaclarke]

Oh you didn't have to do that, I put "no action required". I was just musing on wether that method should even exist. It does exist so feel free to use it.

[gaaclarke]

@walley892 don't forget to fill in the checklist in the description

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Changes reported for pull request #184037 at sha ddde27b