manpage: fix places that still treat FourCCs and pixel formats equal

The video filter chain traditionally used FourCCs for pixel formats.
This was recently changed, but some parts of the manpage were not
updated properly. Now there are two rypes of options: some which take
a FourCC (as used with raw video formats), and some which take a
symbolic format identifier (as used in the video filter chain).

I realize that it's harder to specify FourCC for RGB formats now (TV
stuff may need RGB). They use non-printable characters as part of the
FourCC, and have to be specified as hexadecimal numbers (instead of
a symbolic identifier). Because I can't be bothered to find out what
these numbers are for the respective formats, just remove the old
pseudo-FourCCs from the documentation.
options: also accept ffmpeg pixel format names

Options that take pixel format names now also accept ffmpeg names.
mpv internal names are preferred. We leave this undocumented
intentionally, and may be removed once libswscale stops printing
ffmpeg pixel format names to the terminal (or if we stop passing the
SWS_PRINT_INFO flag to it, which makes it print these).

(We insist on keeping the mpv specific names instead of dropping them
in favor of ffmpeg's name due to NIH, and also because ffmpeg always
appends the endian suffixes "le" and "be".)
vf_format: fix forcing output format

This failed with an assert, because the format of the format of the
output image was not set correctly.