lavc: set the default rc_initial_buffer_occupancy

rc_buffer_size is not set before.

Solve the initial the rate control underflow issue reported in
bug 222.

CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
libx264: use the library specific default rc_initial_buffer_occupancy

By default libav sets it to 3/4 while x264 sets it to 9/10.

CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>