Auto merge of #29745 - mrobinson:compositor-side-scroll-tree, r=mukilan

Add a compositor-side scroll tree

This will allow the compositor to properly chain scrolling requests up
when a node has reached the extent of the scroll area. In addition, it
removes the use of the deprecated WebRender `scroll()` API. This fixes
scrolling on servo.org.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes:
  - Fix #29402.
  - Fix #27996.
  - Fix #27624.
  - Fix #24028.
  - Fix #23918.
  - Fix #21165.
- [x] There are tests for these changes

Cargo.lock

@@ -854,6 +854,7 @@ dependencies = [
  "crossbeam-channel 0.4.4",
  "embedder_traits",
  "euclid",
+ "fnv",
  "gfx_traits",
  "gleam",
  "image 0.24.6",
@@ -5231,6 +5232,7 @@ dependencies = [
  "servo_atoms",
  "servo_url",
  "smallvec",
+ "std_test_override",
  "style_traits",
  "time 0.1.45",
  "uuid",