2D canvas rendering now lives in the compositor, but drawing one canvas
into another still converted the source HTMLCanvasElement into a
DecodedImageFrame in WebContent. That forced a compositor readback for
every drawImage(canvas, ...) call before sending the destination canvas
commands back to the compositor.
Teach the canvas command stream to carry a DrawCanvas command that names
the source canvas surface. The destination 2D context now flushes the
source canvas, records that command, and immediately flushes the
destination command list so the compositor copies the source surface at
the drawImage call boundary. Bitmap sources continue to use DrawBitmap,
and true readback APIs still read pixels explicitly.
Moving 2D canvas rasterization into the Compositor process needs a wire
format for canvas mutations that does not depend on LibWeb state. Add
CanvasCommandList as an apply-once log of drawing operations whose
operands are Gfx value types and whose IPC encoders can be shared by
WebContent and the compositor side.
