ffplay: extend documentation for the -vf option

In particular, make it consistent with the ffmpeg docs.
swr: limit buffer size for discarding.

This prevents insane memory usage in case of insane input values.
Untested due to lack of a testcase that causes such insane allocation

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: limit buffer size for silence injection

This reduces memory usage for unreasonable large silence injections

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