canvas: prune vello scene on each render and make rendering cacheable (#38406)

As noted in #38345, vello scenes only grow. While we can reset them when
clearing viewport (#38356) that is not enough. We need to reset scene on
each render (~each frame) and providing old frame as backdrop to new
scene. Be do this lazily so multiple rendering without any changes
should be cheaper, we still do GPUBuffer mapping, because that would
require more complex work.

Testing: Code functionality is covered by existing WPT tests, but we do
not have any performance test.

---------

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

tests/wpt/meta/html/canvas/element/manual/drawing-images-to-the-canvas/drawimage_html_image.html.ini

@@ -11,3 +11,6 @@
   [Test scenario 12: sx = -20, sy = -20, sw = 50, sh = 50, dx = 20, dy = 20, dw = 125, dh = 125 --- Pixel 70,99 should be light purple.]
     expected: FAIL
 
+  [Test scenario 14: sx = 64, sy = 64, sw = 384, sh = 384, dx = 0, dy = 0, dw = 32, dh = 64 --- Pixel 16,0 should be gray.]
+    expected:
+      if subsuite == "vello_canvas": FAIL

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

@@ -2,4 +2,4 @@
   [putImageData(getImageData(...), ...) has no effect]
     expected:
       if subsuite == "": FAIL
-      if subsuite == "vello_cpu": FAIL
+      if subsuite == "vello_cpu_canvas": FAIL