add more Target Format

[babaq]

Hi,

I have a 16bit gray video encoded with ffv1 in mkv, but when read it's in Gray{FixedPointNumbers.N6f10}, which is a 10bit gray, where the correct type should be N0f16.

The VideoIO.VIO_PIX_FMT_DEF_ELTYPE_LU currently have only a few supported format. In order to support more format combinations, would it better to separate the decoding format and Julia array eltype?

Thanks,
Alex