Hopefully fix crash when the same frame is encoded twice.

Previously if some frame X was passed to the encoder twice
I think this would happen:

X1 written; X stored to _pending_images
X2 written; _pending_images remains the same
X1 encode finishes; X is discarded from _pending_images
Data for X2 is read by the encoder but was just freed

I think this might have resulted in intermittent crashes, which were
fairly common in the 2.17.x branch with the test
ffmpeg_encoder_prores_regression_1

But I didn't conclusively prove it.

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