zmbv has only one function for decoding intra frames, namely
decode_intra; and yet up until now it has been called via a function
pointer. This has been changed.

This also removes spec-incompliant conversions between function pointers
and pointers of type void * and thereby fixes the warning "ISO C forbids
assignment between function pointer and ‘void *’" that GCC emits with
the -pedantic option.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>