Implement get_format

Since raqote doesn't have multiple formats for its surfaces, this is a
noop to avoid crashing with the unimplemented

Cargo.toml

@@ -28,4 +28,3 @@ opt-level = 3
 # Those are here to dedupe winapi since mio is still using winapi 0.2.
 mio = { git = "https://github.com/servo/mio.git", branch = "servo" }
 iovec = { git = "https://github.com/servo/iovec.git", branch = "servo" }
-

components/canvas/raqote_backend.rs

@@ -333,7 +333,7 @@ impl GenericDrawTarget for raqote::DrawTarget {
         raqote::DrawTarget::fill(self, &pb.finish(), pattern.as_raqote(), &draw_options);
     }
     fn get_format(&self) -> SurfaceFormat {
-        unimplemented!();
+        SurfaceFormat::Raqote(())
     }
     fn get_size(&self) -> Size2D<i32> {
         Size2D::new(self.width(), self.height())