Factor perspective scale into filter matrix decomposition

I experimented with passing in a rectangle and computing a geometric
mean of the scale factors for the 4 corners and center. The downside
to that approach is that callers either know the parameter-space bounds
or the device-space bounds. In the latter case, we have to map by the
inverse CTM, which opens up a can of worms. In practice it seemed using
just the center point worked out just as well.

This also updates the sample to draw the axes in the layer space instead
of parameter space. I found this helps display the scale effects of the
parameter-to-layer matrix better.

Bug: skia:9074
Change-Id: I855c85cdbe1072c451aa3a0601571f2e137e5203
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327624
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed