Fix FFmpeg video decoder packet and frame draining

[darioriccio]

Summary

• retry the same AVPacket when avcodec_send_packet reports decoder back-pressure
• drain every frame made available by a packet before reading the next packet
• preserve output ordering when one packet yields multiple frames and use the same path for EOF flushing

Root cause

The previous loop consumed the next demuxed packet after avcodec_send_packet returned EAGAIN, even though FFmpeg requires the caller to receive pending frames and retry the unchanged packet. Inter-frame HEVC streams then lost reference packets and repeatedly emitted Could not find ref with POC, eventually freezing.

Verification

• standalone send/receive probe on the affected HEVC stream: 400/400 frames decoded with the corrected loop, versus 252/400 and reference-frame errors with the previous loop
• integrated macOS Domeport Pro build using the background VideoDecoder path and hardware decoding disabled: an 8K HEVC video advanced continuously, produced distinct rendered frames, and emitted no HEVC POC/reference errors
• EOF drain uses the same multi-frame receive path

This is intentionally limited to the background decoder send/receive contract; it does not change renderer selection or UI behavior.

[jcelerier]

hello ! can you check if this works fine on top of #2121 ? as it's a pretty big refactor of all the video part to enable better performance