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.
Implements spec algorithm for viewport scrolling that first checks if
it's possible to use delta to move the visual viewport before falling
back to scrolling the layout viewport. This is a part of pinch-to-zoom
support.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
