This is implemented by these related changes:
* The Skia alpha type 'Opaque' is selected for surfaces that were
created with the intention of not having an alpha channel.
Previously we were simply creating one with alpha.
* Clearing now happens through Skia's `clear()` which always uses the
source color's value for the result, instead of setting all values
to 0.
* CanvasRenderingContext2D selects a different clearing color based on
the `alpha` context attribute's value.
This reduces the number of `.cpp` files that need to be recompiled when
one of the below header files changes as follows:
CSS/ComputedProperties.h: 1113 -> 49
CSS/ComputedValues.h: 1120 -> 209
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
