[azarouski]

Summary

Fixes in the MJPEG screenshots broadcaster (FBMjpegServer, FBImageProcessor).

• Unchanged frames were re-sent when scaling is enabled. The new screenshot was compared with the last sent frame, which differs from the captured data once it has been scaled, so with mjpegScalingFactor < 100 a static screen was rescaled and streamed at the full framerate. It is now compared with the last captured frame, the scaling factor and the orientation setting.
• Scaled frames were larger than full-size ones. Scaled frames were recompressed with a quality of at least 0.9 regardless of mjpegServerScreenshotQuality. With the default quality of 10 a 50% frame came out bigger than the original. The configured quality is used now.
• Data races. The last frame and the new client flag were accessed from several queues without synchronization.
• First frame for a new client. The last frame is now resent right after the client subscribes, instead of waiting for the 0.5s timer (or up to 20s if the timer fired before the client was subscribed).

Measurements (simulator, static screen)

	Before	After
Frames in 10s at mjpegScalingFactor=50, 10 fps	95 frames, ~10 MB	2 frames, ~54 KB
Time to the first frame for a new client	120–370 ms	1–5 ms

Testing

• FBImageProcessorTests pass.
• Manual runs on the iPhone 16 Pro simulator: frame counts at 50% and 100% scaling, first frame delivery for a late client.