Previously, a transform with a negative scale would result in a
new coordinate system being created when updating the spatial
tree.

This meant that a primitive in that space with a clip in a parent
space would create a clip mask (and was thus unable to be promoted
to a compositor surface).

This change allows negative scales to be part of the same coord
system (since they are still axis-aligned with the parent) and
avoid a clip mask in this case.

Backed out changeset 9040424c0cb8 (bug 1700539) for wpt crashes.
https://hg.mozilla.org/integration/autoland/rev/62e5068e45fd905fdb563245cad4efec1d8c0ac0

Push with failures:
https://treeherder.mozilla.org/jobs?repo=autoland&revision=9040424c0cb887c8defbd51eaa18a235b2b7ceb9&selectedTaskRun=QiRNgmEPQYy6h_9niWzVRw.0

Failure log:
https://treeherder.mozilla.org/logviewer?job_id=334830763&repo=autoland&lineNumber=2951

Unfortunately this doesn't reproduce locally for me on an android emulator build. I'll debug it via logging debug information and doing try runs.

https://hg.mozilla.org/mozilla-central/rev/be0f7947d378

Comment on attachment 9211185 [details]
Bug 1700539 - Allow negative scale in ScaleOffset type.

Beta/Release Uplift Approval Request

• User impact if declined: This patch significantly improves performance of many pages that use a webgl canvas when running webrender. It especially helps for users running sw-wr.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce: If desired, can check via the picture cache debug overlay on the referenced sketchfab page.
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): The patch itself is fairly small, easy to back out if needed. It should only affect content that has a reflection transform, which is rare outside of webgl canvas.
• String changes made/needed:

Comment on attachment 9211185 [details]
Bug 1700539 - Allow negative scale in ScaleOffset type.

Low-risk perf win, approved for 88.0b6.

https://hg.mozilla.org/releases/mozilla-beta/rev/5864b98a2a6e