doc/filters/scale: fix MAN rendering of @ref arguments
doc/filters/scale: mention and link scaler options
doc: move sws_dither option description to scaler.texi

That's the more proper place where the scaler options should be
documented.
fate: add fieldorder test

Signed-off-by: Paul B Mahol <onemda@gmail.com>
swscale: make bilinear scaling the default

Before this commit, sws_init_context() failed with an error if no scaler
was explicitly set.

Defaulting to something reasonable is better behavior.

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