lavfi/vf_colorchannelmixer: replace round by lrint

lavfi/vf_colorchannelmixer: replace round by lrint

lrint is faster here on -ftree-vectorize with GCC. This is likely simply
an artifact of GCC's rather terrible auto-vectorizer, since as per the
instruction set manuals cvtsd2si and cvttsd2si (or their vector equivalents)
have identical cycle timings.

Anyway, regardless of above, lrint is superior to round accuracy wise.

Safety guaranteed as long int has at least 32 bits.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>