These suffixes are annoying when they're redundant, so strip them
automatically. On little endian machines, always strip the "le" suffix,
and on big endian machines vice versa (although I don't think anyone
ever tried to run mpv on a big endian machine).
Since pixel format strings are returned by a certain function and we
can't just change static strings, use a trick to pass a stack buffer
transparently. But this also means the string can't be permanently
stored by the caller, so vf_dlopen.c has to be updated. There seems
to be no other case where this is done, though.
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