Implement a new registration system for filters.

Create a new static array containing pointers to the AVFilter
definitions, so that the non-constant next filter in the AVFilter
struct is not anymore required and the AVFilter definitions may be
stored in shareable memory.

Also change the signature for avfilter_register(), make it return an
int since it may fail if there is not enough space in the static array
for the registered filters.

Originally committed as revision 20605 to svn://svn.ffmpeg.org/ffmpeg/trunk