script: More consistently use f32 and have scrolling methods follow the specification more closely (#39104)

This clarifies the units for scrolling:

 - `f32` is used for internal Servo scrolling APIs as that is the unit
   used in WebRender.
 - `f64` is used for the web-exposed scrolling APIs as that is what the
   WebIDL code generator gives us.

Conversions are done consistently at the boundaries of the two APIs.

In addition, web-exposed scrolling methods are refactored a bit to more
closely follow the specification text. In addition, specification text
is added to those methods so that it is clearer that we are following
it.

Testing: This should not change behavior and is thus covered by existing
tests.

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

components/script/dom/document.rs

@@ -1065,8 +1065,7 @@ impl Document {
             });
 
         if let Some((x, y)) = point {
-            self.window
-                .scroll(x as f64, y as f64, ScrollBehavior::Instant)
+            self.window.scroll(x, y, ScrollBehavior::Instant)
         }
     }