[Impeller] Fix coverage values when a stroked path is rotated

[dnfield]

Fixes flutter/flutter#124131

If the matrix has negative values in it, we were deflating the coverage rect.

[bdero]

LGTM!

[auto-submit]

auto label is removed for flutter/engine, pr: 40917, due to - This commit is not mergeable and has conflicts. Please rebase your PR and fix all the conflicts.

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Changes reported for pull request #40917 at sha 0ab790f

[dnfield]

@gaaclarke - it looks like the playground test draws against a black background, but the golden test against a white one...

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Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #40917 at sha 89e24d7

[gaaclarke]

@gaaclarke - it looks like the playground test draws against a black background, but the golden test against a white one...

It's just how transparent is interpreted between the viewers.