Bug 1746084 - Avoid generating InterpolateMatrix operations if there are no size dependencies. r=hiro

The issue here is that we end up with a transition between mismatched
transform lists that ends up generating an InterpolateMatrix {}
operation. So far so good, but we end up interpolating that a lot of
times and generating an unboundedly-deep operation list.

This implementas an optimization that flattens them to a single matrix
when possible (when there's no dependencies on the containing box).

This is similar to:

  https://chromium.googlesource.com/chromium/src.git/+/2b89cc4df436e672ef9cf940d1c0dc73fef82a4a

We fix the to_pixel_length() behavior for LenghtPercentage to be
correct (and update callers to preserve behavior).

Differential Revision: https://phabricator.services.mozilla.com/D134784