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/offscreencanvas.rs

@@ -72,8 +72,11 @@ impl OffscreenCanvas {
         )
     }
 
-    pub(crate) fn get_size(&self) -> Size2D<u64> {
-        Size2D::new(self.Width(), self.Height())
+    pub(crate) fn get_size(&self) -> Size2D<u32> {
+        Size2D::new(
+            self.Width().try_into().unwrap_or(u32::MAX),
+            self.Height().try_into().unwrap_or(u32::MAX),
+        )
     }
 
     pub(crate) fn origin_is_clean(&self) -> bool {