skip reencoding compatible video streams

[HookedBehemoth]

Since MP4 already supports h264 and webm already supports vp8 & vp9 pretty much everywhere, reencoding them can be skipped. Tested on my own files. When seeking, sometimes audio cuts out for a second as -c:v copy only forks on full chunks.

I have never used GO before and the change to the go.sum file happened through one of the setup commands. I'll remove it if you don't want that in the tree.