[Impeller] Use Matrix::MakeTranslateScale to compute the source-to-destination mapping in DrawImageRect

[jason-simmons]

No description provided.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

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[jonahwilliams]

LGTM