extend log line buffer sizes

The string passed to av_vlog() from libx264's X264_log overflows the 512
byte part_size, truncating x264 parameter output.  Double this size
and put it in a #define so it only needs to be changed in one place.

Signed-off-by: Wolfram Gloger <video06@malloc.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>