avformat/mpc: attempt to allocate a packet that is not smaller than the data inside it

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: add prototypes for resample dsp functions

Should fix compilation failures with MSVC and any other compiler
without inline asm support.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>