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>