Backend handoff reworked for remote ImageBuffers

Component: WebKit GPU Process / Remote Rendering | 1a3eac1

Accelerated canvas and WebGL rendering is split across processes: the WebProcess holds a RemoteImageBufferProxy while the actual pixel backing store — often a GPU-backed IOSurface — lives in the GPU process behind RemoteRenderingBackend and RemoteImageBuffer, with IPC keeping the two synchronized. IOSurfaces are pooled and recycled for performance, so any code path holding a live reference to one — including transferring its handle to the WebProcess — prevents reuse.

Previously, every new GPU-process-backed ImageBuffer sent its backend handle to the WebProcess on creation as an async DidCreateBackend message, and RemoteImageBufferProxy started backend-less, blocking dependent operations until that message arrived. This commit replaces that handshake: the backend is instantiated at creation time, and the underlying handle is fetched synchronously only when a caller actually needs it — which is now just PlaceholderRenderingContext and view transitions.

  Before:                              After:
  create ImageBuffer                   create ImageBuffer
    └─► GPU: build backend               └─► GPU: build backend (eager)
          └─► async DidCreateBackend            proxy usable immediately
                └─► WebProcess gets handle
                      ▲ pins IOSurface     handle fetched synchronously
                        defeats pool       only by the two consumers
                      ▲ callers must       that need it
                        drain the message

Significance

Eagerly shipping handles to the WebProcess on every buffer creation pinned IOSurfaces and defeated the IOSurfacePool, causing alloc/dealloc churn under tight self-drawImage() loops on accelerated 2D canvas. Beyond the performance fix, this removes an entire class of async IPC handshake that callers previously had to explicitly drain — a state machine where an object exists but is not yet usable is a recurring source of ordering bugs, and that state no longer exists here.

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