Filter NALs globally

[deiteris]

Handle NAL filtering globally and remove video buffer copy caused by it from Linux code.

[nowrep]

filter_NAL needs to stay because radeon vaapi includes AUD and x264 includes SEI.
But it may be possible to just use offset into the encoded buffer instead of doing a copy?

[deiteris]

Since ClientConnection already implements header NALs extraction, maybe filtering part could be moved there so it's consistent across all implementations.

[deiteris]

Only after I've reworked filtering code I've realized why it actually doesn't work properly with AUD. AUD is the first NAL that we may encounter. This results in configuration headers never sent because they're not detected, and that's why stream may not work properly when it's present.

Ideally, this should be resolved by global configuration headers as it's done in #1385. For now, just skipping AUD, if it's the first header, is enough. SEI NALs will work as is.

[zmerp]

Overall good to me, but I would personally remove the temporary b and l variables in processH264Nals and processH265Nals.

[deiteris]

I think this was only tested with AMD VA-API. If someone could test AMF and NVEnc with recent changes and confirm it works properly, that would be great.

[nowrep]

Works fine with AMF.

[deiteris]

Ready to merge.