Recording (Windows): keep the video on the clock when the writer misses ticks

[EtienneLescot]

Part of #920 (rank 6). Effort S, risk low. Waits for #929 and #931.

Problem

• The Windows sink writer renumbers the screen video at a constant rate: 1449 frames at exactly 16.667 ms (measured), whatever timestamps it is given.
• When the writer misses a tick under load, writeVideoFrames resyncs to now without writing the missed slots (main.cpp). The video therefore loses one frame of time per missed tick, while the audio follows the clock: on a slow machine the picture drifts ahead of the sound ([Bug]: Recorded video game is not played properly #510).

Proposal

• On a late tick, write one repeated frame per missed slot so the frame count keeps up with the clock. perf(windows): repeat the last frame instead of reading back an unchanged screen #931's repeatLastVideoSample makes a repeat cost nothing, which is why this waits for it.

Verify

• A flash-and-beep take under CPU load: the image/sound offset stays flat from start to end instead of growing. [pacing] frames/elapsed equals the fps.