GitLab

This implementation brings in several improvements for blending:
- it's done faster, due to reducing the number of readbacks.
- the backdrop doesn't suffer any transformation, so the result is more
  correct.
- the results are similar to the ones obtained using the software paths.
- blending is always applied after filters, even when the filters
  can be expressed using a color matrix.

The initial experiment: https://codereview.chromium.org/555133002/.

BUG=243223

Review URL: https://codereview.chromium.org/658483003

Cr-Commit-Position: refs/heads/master@{#300657}