libavcodec/libx264: fix reference frame computation based on level

libavcodec/libx264: fix reference frame computation based on level

The current implementation allows passing levels to libavcodec as
integers (such as "31" instead of "3.1").

However, in this case, the maximum reference frame value per level was
ignored because libavcodec converted the string to 310 instead of 31.

Since libx264 has correctly parsed the level to int
(x4->params.i_level_idc), we should rely on this value instead of
attempting to parse the level string on our own.

Signed-off-by: Josh Brewster <josh.brewster@protonmail.com>
Signed-off-by: Josh de Kock <josh@itanimul.li>