[Impeller] The RRect blur fast path should only apply for solid color draws

[bdero]

Canvas::AttemptDrawBlurredRRect should return false when the paint's ColorSource is not solid color.

It might be worth adding an ad-hoc ColorSource enum to the paint state for optimizations like this to use.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.