canvas: Clear vello scene if possible (#38356)

Vello scene only ever grows, so we need to clear it as soon as it's
possible (in clear rect). This PR also adds ignore_clips to
vello_backend (already exists in vello_cpu_backend).

Testing: Behavior is verified by existing tests, as this is mainly a
change for performance. There are currently no performance tests. This
makes bunnymark actually playable (from 3 FPS to 20 FPS on vello_cpu).

---------

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

tests/wpt/meta/html/canvas/element/path-objects/2d.path.clip.winding.evenodd.1.html.ini

@@ -0,0 +1,4 @@
+[2d.path.clip.winding.evenodd.1.html]
+  [evenodd winding number rule works in clip]
+    expected:
+      if subsuite == "vello_canvas": FAIL

tests/wpt/meta/html/canvas/element/path-objects/2d.path.clip.winding.evenodd.2.html.ini

@@ -0,0 +1,4 @@
+[2d.path.clip.winding.evenodd.2.html]
+  [evenodd winding number rule works in clip]
+    expected:
+      if subsuite == "vello_canvas": FAIL

tests/wpt/meta/html/canvas/element/pixel-manipulation/2d.imageData.put.clip.html.ini

@@ -1,5 +0,0 @@
-[2d.imageData.put.clip.html]
-  [putImageData() is not affected by clipping regions]
-    bug: https://github.com/linebender/vello/issues/1088
-    expected:
-      if subsuite == "vello_canvas": FAIL

tests/wpt/meta/html/canvas/element/pixel-manipulation/2d.imageData.put.unchanged.html.ini

@@ -1,3 +1,5 @@
 [2d.imageData.put.unchanged.html]
   [putImageData(getImageData(...), ...) has no effect]
-    expected: FAIL
+    expected:
+      if subsuite == "": FAIL
+      if subsuite == "vello_cpu": FAIL