Reduce iOS video enc/dec redundant code, add more H.265 QP parsing logic

[Piasy]

No description provided.

[taste1981]

@Piasy Thanks for you patch. I'm adding @jianjunz to review this.

[jianjunz]

Thanks for your change. One reason of having both RTCVideoDecoderH264.mm and RTCVideoDecoderH265.mm is HEVC is not included in upstream libwebrtc. Making too many changes to RTCVideoDecoderH264.mm will increase the effort during rebasing. So I would prefer to keep RTCVideoDecoderH264.mm unchanged. As this is a trade off, I'm open to merge these two files if it brings more benefits.

[Piasy]

Hi @jianjunz , as you can see in the diff, the modification to RTCVideoEncoder/DecoderH264 isn't very large, only several #if #else #endif, I don't think it will increase much effort for future rebasing. And if we use separate files for H265, if upstream makes any update or improvement to H264 encoder/decoder, it's not easy to notice them, and apply those changes manually would be annoying.