avfilter/vf_scale: add ov/hsub

Suggested-by: divVerent
Reviewed-by: Stefano
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil: add av_fopen_utf8()

fopen() on windows uses UTF-16, we use UTF-8 everywhere, this
function bridges the gap by using avpriv_open()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>