canvas: Add vello_cpu backend (#38282)

vello_cpu does not have any tests timeouts, because we do not need
download stuff from GPU as all work happens on CPU. So performance wise
it's better then classic vello at least for our usecase. There are some
vello bugs, but I think we will be able to sort them out within
upstream, eventually. Interestingly enough there are no new PASS like
they were with classic vello.

Difference with raqote can be observed here:
https://github.com/sagudev/servo/actions/runs/16549241085/attempts/1#summary-46802486798

## Known vello problems:

- https://github.com/linebender/vello/issues/1119
- 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`
- `putImageData(getImageData(...), ...)` is lossy (precision problems,
might be due to ImageData being unmultiplied)
-
`/html/canvas/element/pixel-manipulation/2d.imageData.put.unchanged.html`

Testing: Tested using vello_cpu_canvas subsuite

---------

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

python/servo/try_parser.py

@@ -125,6 +125,19 @@ def handle_preset(s: str) -> Optional[JobConfig]:
             unit_tests=False,
             number_of_wpt_chunks=2,
         )
+    elif any(word in s for word in ["vello-cpu", "vello_cpu"]):
+        return JobConfig(
+            "Vello-CPU WPT",
+            Workflow.LINUX,
+            wpt=True,
+            wpt_args=" ".join(
+                [
+                    "--subsuite-file ./tests/wpt/vello_cpu_canvas_subsuite.json",
+                    "--subsuite vello_cpu_canvas",
+                ]
+            ),
+            build_args="--features 'vello_cpu'",
+        )
     elif any(word in s for word in ["vello"]):
         return JobConfig(
             "Vello WPT",