swresample: Choose 16bit internally only if input and output is 16bit or less

or if no rematrix and no resampling is performed and the input is 16bit
note reampling and rematrix itself always use more than 16bit internally
the "internal" sampling format is the format between these steps

Its unlikely the difference from this commit is audible in any case
unless there is some bug either before or after the change.
but multiple people prefer this and it slightly improves the precission
of computations.

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