Blend mode support

[k-o-d-e-n]

No description provided.

[calda]

Thanks for the contribution @k-o-d-e-n! I'm going to convert this to a draft for now since CI isn't passing, but feel free to continue working on this. Once CI passes, I think we can merge this.

• I think the build is probably failing because the new BlendMode+Filter.swift file isn't referenced in the xcode project.
• You'll need to run the snapshot test suite to create snapshot images for the sample asset you added

[calda]

@k-o-d-e-n, some of the checks are failing. I think you need to:

• regenerate the snapshot images for the samples you added, to resolve the Snapshot "Nonanimating/blend_mode_test (0%)" does not match reference error. You can do that by either deleting the file on disk and having it be recreated on the next run, or by adding isRecording = true to the testCoreAnimationEngine() test case and rerunning the test
• run the Swift liner (bundle exec rake format:swift)

[calda]

If the snapshot test case keeps failing, then the rendered output must be slightly nondeterministic. You an work around this type of issue by decreasing the comparison precision (example: https://github.com/airbnb/lottie-ios/blob/master/Tests/SnapshotConfiguration.swift#L43)

[k-o-d-e-n]

I have no failed tests with blend_mode_test.json. I don't know why it fails on CI.

[calda]

I pushed some commits that address the CI failures.

• For the Typeface/B snapshots, I regenerated them using isRecording = true. Looks like that animation has a blend mode setup so its snapshots were affected by this PR.
• For the blend_mode_test snapshots, it looks like those were being rendered nondeterministically based on the environment running the tests (we've hit this a few times before). I fixed this by decreasing the comparison precision for that test to 0.99

Will merge once checks pass. Thanks again for your contribution!

[k-o-d-e-n]

Thanks for help!