ffplay: add support for interactive volume control

This is a feature heavily inspired by the mpv player. At the moment, methods
for adjusting volume in ffplay are rather clumsy: either one needs to set it
system-wide, or one needs to set it via the volume filter.

This patch adds key bindings identical to the mpv defaults for muting/unmuting
and increasing/decreasing the volume interactively without any introduction of
external dependencies.

TODO: doc update, possible mouse button bindings (mpv has this).
Signed-off-by: NGanesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: NMarton Balint <cus@passwd.hu>

ffplay: dynamically allocate filename buffer

filename was set to an arbitrary 1024 characters. ffplay would thus be unable to
play files whose name exceeds that arbitrary threshold.

This patch dynamically allocates and frees the filename buffer to remove such
limitations.
Signed-off-by: NGanesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: NMarton Balint <cus@passwd.hu>