In addition, this does not allow creating frames contexts with sw_format
for which no known transfer formats exist. In theory, we should check
whether the chroma format (i.e. the sw_format) is supported at all by
the vdpau driver, but checking for transfer formats has the same effect.

Note that the pre-existing code adds 1 to priv->nb_pix_fmts[i] for
unknown reason, and some checks need to account for that to check for
empty lists. They are not off-by-one errors.