Initial move of canvas/context/snapshot size from u64 -> u32

Changed the two instances of `euclid::default::Size2D<u64>` in
`servo/components/script/canvas_context.rs` and
`servo/components/shared/snapshot/lib.rs` that were outlined as the bare minimum
in "Make canvas/context/snapshot size be u32 everywhere" to
`euclid::default::Size2D<u32>`. Every other change made in this commit is
either:
 - of similar nature, and is the minimum that would allow compilation
 - resolving lints triggered by the former

More might be needed to complete the issue, but I feel like this is a good
starting point.

This commit includes changes to the following components:
 - canvas
 - pixels
 - script
 - shared/canvas
 - shared/snapshot

Signed-off-by: hashcatHitman <155700084+hashcatHitman@users.noreply.github.com>

components/script/dom/webxr/xrwebgllayer.rs

@@ -129,10 +129,7 @@ impl XRWebGLLayer {
                 HTMLCanvasElementOrOffscreenCanvas::HTMLCanvasElement(canvas) => canvas.get_size(),
                 HTMLCanvasElementOrOffscreenCanvas::OffscreenCanvas(canvas) => {
                     let size = canvas.get_size();
-                    Size2D::new(
-                        size.width.try_into().unwrap_or(0),
-                        size.height.try_into().unwrap_or(0),
-                    )
+                    Size2D::new(size.width, size.height)
                 },
             };
             Size2D::from_untyped(size)