canvas: Gate raqote backend behind feature (enabled for now) (#38312)

We want to eventually remove raqote backend, but for now we can gate it
behind a feature (still enabled by default in servoshell) like the rest
of backends. `dom_canvas_backend=auto` will select first available
backend. Builds on top of https://github.com/servo/servo/pull/38310 to
support cases where no backend is available.

Testing: It compiles with or without feature

---------

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

components/canvas/lib.rs

@@ -5,11 +5,16 @@
 #![deny(unsafe_code)]
 
 mod backend;
+
+#[cfg(feature = "raqote")]
+mod raqote_backend;
+
 #[cfg(any(feature = "vello", feature = "vello_cpu"))]
 mod peniko_conversions;
-mod raqote_backend;
+
 #[cfg(feature = "vello")]
 mod vello_backend;
+
 #[cfg(feature = "vello_cpu")]
 mod vello_cpu_backend;