Use pixel diffs in Vello Tests

Nobody has claimed this yet.

We're currently using `nv-flip` for Vello Classic's tests, but this doesn't work well for 2d imaging screenshots.

As such, we've moved away from it in Masonry - https://github.com/linebender/xilem/pull/904, and here for the new sparse strip tests.

We should do the same for Vello Classic.

The main thing which would need to change is the code around here:

https://github.com/linebender/vello/blob/a97dddc9655c576d72e563a4c633efecaf2315ca/vello_tests/src/snapshot.rs#L62-L80

We don't need to make this overly configurable; we just want a threshold which all the current tests pass.
We plan to merge the Vello Classic and Sparse Strips test suites in the foreseeable future, so a quick-and-dirty solution here is fine.

The diff algorithm in Vello Sparse Strips can be found at:
https://github.com/linebender/vello/blob/a97dddc9655c576d72e563a4c633efecaf2315ca/sparse_strips/vello_sparse_tests/tests/util.rs#L443-L511

Start in vello_tests/src/snapshot.rs around the current nv-flip comparison, then read the pixel-diff algorithm in sparse_strips/vello_sparse_tests/tests/util.rs. Replace the Vello Classic comparison with a threshold-based pixel diff, choosing a threshold that lets all current tests pass. Run the Vello Classic test suite to verify the snapshots.

Written by the indexing model from the issue text.