Fetch the image and its RAW region under one buffer lock

[mikelangmayr]

• read_frame locked, fetched and unlocked in one call, so fetching the image and the RAW region meant two locks with a window between them where a new frame could land, leaving the two halves describing different exposures
• Splits the fetch body out behind lock_newest_buffer and fetch_region, leaving read_frame behaving exactly as before
• Adds raw pair, which holds one lock across both fetches and dispatches them with the same frame number
• It reads whatever buffer is newest rather than triggering an exposure, so it also reads out a frame the controller already holds
• Refuses when the mode has more than one detector, rather than dispatching part of a frame
• Verified on the emulator, and against an Archon where it wrote a 8256x4120 image and its matched raw capture, both carrying frame 24

[prkrtg]

Should we first check if (error == NO_ERROR) before attempting to unlock the buffer?

[mikelangmayr]

Yes, done. UNLOCK after a failed fetch would eat the unread block data as its reply, and the next LOCKn replaces the lock anyway.