please remove 4:4:0 in vp10

There is no legitimate use case for 4:4:0, it seems like an engineering feat 
where x/y chroma subsampling are coded as independent variables and so it just 
happened that 4:4:0 was created along with 4:2:2 and 4:4:4. Nobody uses it, 
nobody will ever use and nobody wants it. Please remove it in vp10, it's a 
support burden for external implementations.

If possible, also consider removing/deprecating it in vp9, although I see that 
might be difficult since the bitstream format is already stable. E.g., you 
could consider making the chroma_subsampling combination x=0 and y=1 illegal in 
future releases of the codec.

Bug report based on feedback from FFmpeg community after patch submission for 
profile 1/2/3, they are not amused about 4:4:0 and the required extra code in 
FFmpeg to support it. Here's a quote from IRC to support this bug report:

[12:28pm] <wm4> they are probably fascinated by the idea that they can 
implement different subsampling so generically and easily
[12:28pm] <wm4> but that doesn't mean it's a good idea to allow such 
obscure/useless formats

Original issue reported on code.google.com by rsbul...@gmail.com on 6 May 2015 at 4:35

gshORTON / webm

please remove 4:4:0 in vp10 #996