Adds pinch event handling that adjusts the VisualViewport scale and
offset. VisualViewport's (offset, scale) is then used to construct a
transformation matrix which is applied before display list execution.
Before this change we would emit PushStackingContext/PopStackingContext
display list items regardless of whether the stacking context had any
transform/opacity/clip effects.
Display list size on https://x.com/ladybirdbrowser is reduced from ~2700
to ~800 items.
When rounding a CSSPixelRect to a DevicePixelRect, we simply pulled its
width and height through round() and called it a day. Unfortunately this
could negatively affect the rect's perceived positioning.
A rect at { 0.5, 0.0 } with size { 19.5 x 20.0 } should have its right
edge at position 20, but after rounding it would end up at { 1, 0 } with
size { 20 x 20 }, causing its right edge to be at position 21 instead.
Fix this by first rounding the right and bottom edges of the input rect,
and then determining the dimensions by subtracting its rounded position.
Fixes#245.
