canvas: Use vello_cpu as default canvas backend (#38844)

We really want to remove font-kit from dep tree, so this is the first
step into removing raqote from servo. While vello_cpu is not perfect
replacement, I am confident that we will resolve all issues eventually:
https://github.com/servo/servo/issues/38345 (most important ones already
have PRs).

Reviewable per commit.

Testing: Existing WPT tests.
Try run: https://github.com/sagudev/servo/actions/runs/17138369290

---------

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

tests/wpt/meta/html/canvas/element/fill-and-stroke-styles/2d.gradient.interpolate.coloralpha.html.ini

@@ -1,6 +1,4 @@
 [2d.gradient.interpolate.coloralpha.html]
   [Canvas test: 2d.gradient.interpolate.coloralpha]
     bug: https://github.com/linebender/vello/issues/1056
-    expected:
-      if subsuite == "vello_canvas": FAIL
-      if subsuite == "vello_cpu_canvas": FAIL
+    expected: FAIL

tests/wpt/meta/html/canvas/element/fill-and-stroke-styles/2d.gradient.radial.cone.behind.html.ini

@@ -1,4 +0,0 @@
-[2d.gradient.radial.cone.behind.html]
-  [Canvas test: 2d.gradient.radial.cone.behind]
-    expected:
-      if subsuite == "": FAIL

tests/wpt/meta/html/canvas/element/fill-and-stroke-styles/2d.gradient.radial.cone.shape2.html.ini

@@ -1,4 +0,0 @@
-[2d.gradient.radial.cone.shape2.html]
-  [Canvas test: 2d.gradient.radial.cone.shape2]
-    expected:
-      if subsuite == "": FAIL

tests/wpt/meta/html/canvas/element/fill-and-stroke-styles/2d.gradient.radial.outside2.html.ini

@@ -1,4 +0,0 @@
-[2d.gradient.radial.outside2.html]
-  [Canvas test: 2d.gradient.radial.outside2]
-    expected:
-      if subsuite == "": FAIL

tests/wpt/meta/html/canvas/element/fill-and-stroke-styles/2d.gradient.radial.outside3.html.ini

@@ -1,4 +0,0 @@
-[2d.gradient.radial.outside3.html]
-  [Canvas test: 2d.gradient.radial.outside3]
-    expected:
-      if subsuite == "": FAIL