`webview.paint()` dispatches a render command to Servo's paint thread
asynchronously, so the subsequent `read_to_image()` raced the paint
thread and reliably returned cleared-white pixels on data: URLs (T10.8).
Clear `has_pending_frame` before dispatching paint, then spin the Servo
event loop until `notify_new_frame_ready` re-arms it or 32 ms elapse
(override via `ELY_PAINT_BARRIER_MS`). Bench: software path now ships
real RGBA bytes for 60 frames instead of all-white.
ZacharyZhang-NY