demux_raw: change license to LGPL

All relevant authors have agreed.

I'm removing the NV12 FourCC, which was added in f910f3d9 by someone who
was not contacted for the relicensing. I doubt the remaining code is
copyrightable (basically all what remains the fact is that NV12 uses the
same amount of space like YV12), but in this case I feel more
comfortable removing it.

Might contain some trace amounts of "michael"'s copyright, who agrees
with LGPL only once the core is relicensed - but with the core already
mostly relicensed, I'm changing the license header to LGPL, and only
marking this in the "Copyright" file.