vf: add vf_convert as interim replacement for vf_scale

We want to drop vf_scale, but we still need a way to auto convert
between imgfmts. In particular, vf.c will auto insert the "scale" filter
if the VO doesn't support a pixfmt.

To avoid chaos, create a new vf_convert.c filter, based on vf_scale.c,
but without the unrelicensed code parts. In particular, this filter does
not do scaling and has no options. It merely converts from one imgfmt to
another, if needed.