canvas: Add vello backend (#36821)

Add vello backend by implementing Backend traits in canvas crate (so
this lives in canvas_paint_thread - embedded process). Current
implementation uses normal wgpu, so we block on GPU work. Vello backend
is gated behind `vello` feature and `dom_canvas_vello_enabled` pref.

Feature-wise this backend is on on par with raqote (sometimes better
sometimes worse), but performance wise it's worse.

## Known vello problems:

- image roundtrip does not work (fixed in
https://github.com/linebender/vello/pull/974)
- https://github.com/linebender/vello/issues/1066 (fixed)
- clip layers are not working properly:
https://github.com/linebender/vello/issues/1061
  - `/html/canvas/element/pixel-manipulation/2d.imageData.put.*`
  - `/html/canvas/element/path-objects/2d.path.clip.intersect.html`
- https://github.com/linebender/vello/issues/1056
-
`/html/canvas/element/fill-and-stroke-styles/2d.gradient.interpolate.coloralpha.html`
- `kurbo::Cap::Butt` is defect (only visible with big lineWidth)
https://github.com/linebender/vello/issues/1063
  - `/html/canvas/element/line-styles/2d.line.cross.html`
  - `/html/canvas/element/line-styles/2d.line.miter.acute.html`
- other lack of strong correct problems
(https://github.com/linebender/vello/issues/1063#issuecomment-2998084736):
  - `/html/canvas/element/path-objects/2d.path.rect.selfintersect.html`
- There is currently no way to do put image properly in vello as we
would need to ignore all clips and other stuff (we try to work around
this on best effort basis)
https://github.com/linebender/vello/issues/1088
  - `/html/canvas/element/pixel-manipulation/2d.imageData.put.*`
- precision problems
  - `/html/canvas/element/path-objects/2d.path.stroke.scale2.html`
  - `/html/canvas/element/path-objects/2d.path.arc.scale.1.html`

## Known servo problems

- bad performance due to blocking on GPU work
  - some get/put intensive tests `TIMEOUT`
- proper shadow support (non-blocker as we already are living without it
now)
- support for rect shadow is there but unimplemented currently as that's
the state in raqote

Testing: `mach try vello` will run normal WPT (with raqote) +
vello_canvas subsuite that runs only on `/html/canvas/element`. All
subsuite expectations are stored separately.
Fixes: #36823
Fixes: #35230

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

python/servo/try_parser.py

@@ -104,7 +104,7 @@ def handle_preset(s: str) -> Optional[JobConfig]:
         return JobConfig(
             "WebGPU CTS",
             Workflow.LINUX,
-            wpt=True,  # reftests are mode for new layout
+            wpt=True,
             wpt_args="_webgpu",  # run only webgpu cts
             profile="production",  # WebGPU works to slow with debug assert
             unit_tests=False,
@@ -125,6 +125,20 @@ def handle_preset(s: str) -> Optional[JobConfig]:
             unit_tests=False,
             number_of_wpt_chunks=2,
         )
+    elif any(word in s for word in ["vello"]):
+        return JobConfig(
+            "Vello WPT",
+            Workflow.LINUX,
+            wpt=True,
+            wpt_args=" ".join(
+                [
+                    "--subsuite-file ./tests/wpt/vello_canvas_subsuite.json",
+                    "--subsuite vello_canvas",
+                    "--processes 1",
+                ]
+            ),
+            build_args="--features 'vello'",
+        )
     elif any(word in s for word in ["lint", "tidy"]):
         return JobConfig("Lint", Workflow.LINT)
     else: