vf_scale: replace ancient fallback image format selection

If video output and VO don't support the same format, a conversion
filter needs to be insert. Since a VO can support multiple formats, and
the filter chain also can deal with multiple formats, you basically have
to pick from a huge matrix of possible conversions.

The old MPlayer code had a quite naive algorithm: it first checked
whether any conversion from the list of preferred conversions matched,
and if not, it was falling back on checking a hardcoded list of output
formats (more or less sorted by quality). This had some unintended side-
effects, like not using obvious "replacement" formats, selecting the
wrong colorspace, selecting a bit depth that is too high or too low, and
more.

Use avcodec_find_best_pix_fmt_of_list() provided by FFmpeg instead. This
function was made for this purpose, and should select the "best" format.

Libav provides a similar function, but with a different name - there is
a function with the same name in FFmpeg, but it has different semantics
(I'm not sure if Libav or FFmpeg fucked up here).

This also removes handling of VFCAP_CSP_SUPPORTED vs.
VFCAP_CSP_SUPPORTED_BY_HW, which has no meaning anymore, except possibly
for filter chains with multiple scale filters.

Fixes #1494.

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