h264: Fix DTS Extractor when single SEI NALU is received

[gkubes]

bluenviron/mediamtx#3614

On a TP-Link Tapo C120 camera, an AU is received with a single SEI NALU. This causes issues within the DTS_Extractor because neither an IDR or non-IDR NALU is received.

This change returns the previously calculated DTS value in the case that only NALUs that are not necessary to reconstruct the reference pictures are received as defined by the nal_ref_idc bits indicate.

[gkubes]

@aler9 looks like you updated the github actions config just after I posed this pull request. Do I need to resubmit this request to get the 2 workflows to run?

[aler9]

Hello, i'm not particularly convinced about the nal_ref_id - based algorithm, but, since this does not conflict with any existing unit test, and since you provided an additional unit test that may allow to change the algorithm in the future without breaking compatibility with the TP-Link camera, i think this can be merged. Thanks!