present() now snapshots the PaintingSurface into an ImmutableBitmap
and publishes it to the ExternalContentSource, so the rendering thread
never touches the live GPU surface — eliminating the data race
described in the ExternalContentSource commit (problem 1).
Canvas elements are registered with Page and presented once per frame
from the event loop, rather than on every individual draw call in
CRC2D::did_draw(). A dirty flag on HTMLCanvasElement ensures the
snapshot is only taken when content has actually changed, and makes
the present() call in CanvasPaintable::paint() a no-op when the
surface has already been snapshotted for the current frame.
PaintContext dates back to a time when display lists didn't exist and it
truly represented "paint context". Renaming it to better align with its
current role.
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
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
