avfilter/formats: Avoid allocations when merging formats and samplerates

avfilter/formats: Avoid allocations when merging formats and samplerates

This is the analogue of cfc65520324ae640299bd321ef88ae76dcee6f78 for
formats and samplerates; in contrast to said commit, one can avoid
allocating a new array for formats as well (the complications of the
generic channel layouts made this impossible for channel layouts).

This commit also starts to move the line continuation '\' chars to the
left to keep them in line with MERGE_REF() as well as with the 80 lines
limit.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>