2D canvas contexts started publishing partial frames after canvas
rasterization moved into the Compositor. WebContent still splits large
recorded command lists after 64 commands, but every split batch was sent
through the same compositor path as the end-of-frame flush. The
compositor replayed each batch into the DrawCanvas source surface,
so a pending present could sample a canvas after clear and before the
rest of the next frame had been drawn. Canvas-heavy pages such as
slither.com then flickered between partial and complete frames.
Carry an explicit commit bit with 2D canvas command updates. Non-commit
batches now update a hidden working canvas in the Compositor, while the
display-list-visible surface keeps the last committed canvas contents.
The end-of-frame canvas preparation sends the commit boundary, including
the empty-commit case needed when the auto-flush consumed all recorded
commands before prepare_for_compositing() runs.
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.
The display list can now refer to canvas ids, but WebContent still had
no channel for creating or updating those canvas resources in the
Compositor. Both 2D and WebGL canvases would have had to grow the IPC
plumbing in the same commit that changes the rendering contexts.
This adds the Compositor-side CanvasHost, WebContent transport objects,
and the IPC/CMake pieces needed to allocate, update, read back, and
destroy remote canvas contexts. The rendering contexts are not switched
over yet, keeping this as plumbing for later commits.
