Add sync_id to tile messages

[pford]

With parallel execution of message handling, there can be multiple data streams from the backend for sending raster tile data, causing render errors in the frontend during zooming and animation, especially for large images. The sequence for each stream is RASTER_TILE_SYNC (end_sync = false) -> RASTER_TILE_DATA -> RASTER_TILE_SYNC (end_sync = true). We need to add a sync_id so that the streams can be kept consistent.