[WIP] Removal of ClipRegion parameter to StackingContext

[kvark]

This is a rough PR, since I had to change the reftests...
filter-grayscale is the most annoying one, where it seems like the child stacking context's bounds are not consistently treated in the same space. Likely related to #859

cc @mrobinson @glennw

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This change is 

[mrobinson]

This might be failing because of the tidy failures:

1.12s$ servo-tidy
Checking the config file...
  Progress: 11% (8/68)Checking files for tidiness...
  Progress: 97% (66/68)./wrench/src/yaml_frame_reader.rs:520: Line is longer than 120 characters
  Progress: 98% (67/68)  Progress: 100% (68/68)
Running the dependency licensing lint...

I don't understand exactly why this change would require a change to reference tests, but I'm going to take a closer look today.

[mrobinson]

It seems like this test doesn't actually have to change? If you are trying to test a different behavior, maybe it would be better to rely only on a new test?

[bors-servo]

☔ The latest upstream changes (presumably #866) made this pull request unmergeable. Please resolve the merge conflicts.

[glennw]

@kvark What's the status of this PR?

[kvark]

Closing for not being able to finish it in time, and @mrobinson will likely do this more efficiently.