video/external: Reorder frames decoded by OpenH264 manually

[torokati44]

This should unblock #18581, despite cisco/openh264#3837.

[torokati44]

Hm, I wonder if instead we should use the composition time of the decoded frames (assuming OpenH264 carries them over correctly from the encoded ones, but surely it does), to govern the "lead time" with which the FLV streaming feeds the decoder... 🤔
Then we could better match FP even, in which frame is presented when, getting the delay lower than it currently has to be.

[torokati44]

... and/or separate the "here's an encoded chunk" and "gimme a decoded frame" calls, like they are in FFmpeg (libavcodec), so they don't have to be 1:1 with each other...

[torokati44]

Note to self: if we go forward with this, then TODO clear the frame queue on seek (especially on backward ones), decide whether it's signalled explicitly from the outside or detected based on frame_ids passed in, and see how those work with FLV sources