Bug xxxxxx - Move the RoundToInt functions to be standalone in the gfx headers. r=
Bug xxxxxx - Introduce a ScaleFactor struct and use it to type conversions between units. r=
Bug xxxxxx - Replace a couple of Rect scaling functions with strongly typed operators. r=
Bug xxxxxx - Convert WidgetSpaceToCompensatedViewportSpace to the new gfx types. r=
Bug xxxxxx - Remove ToCSSIntRectRoundIn and convert APZC::CalculateResolution to return a ScaleFactor. r=