Simplify the "viewport adjustment" for feImage

This is essentially a scale operation after moving the origin to the
upper-left corner of the destination rectangle.

Make ComputeViewportAdjustmentTransform() return a scale factor instead
and rename it to ComputeViewportAdjustmentScale().

Bug: 1411691
Change-Id: Ic15aef9135555caa01bccee21ea5fe320f3c044a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4860125
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/main@{#1198318}