Rename IOCompositor to Paint (#41176)

For a long time, the "Compositor" hasn't done any compositing. This is
handled by WebRender. In addition the "Compositor" does many other
tasks. This change renames `IOCompositor` to `Paint`.

`Paint` is Servo's paint subsystem and contains multiple `Painter`s.
This change does not rename the crate; that will be done in a
followup change.

Testing: This just renames types and updates comments, so no new tests
are necessary.

---------

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/html/htmlmetaelement.rs

@@ -144,7 +144,7 @@ impl HTMLMetaElement {
 
         if let Ok(viewport) = ViewportDescription::from_str(&content.value()) {
             self.owner_window()
-                .compositor_api()
+                .paint_api()
                 .viewport(self.owner_window().webview_id(), viewport);
         }
     }