there already is a function, av_clip_uintp2() that clips a signed integer
to an unsigned power-of-two range, i.e. 0,2^p-1

this patch adds a function av_clip_intp2() that clips a signed integer
to a signed power-of-two range, i.e. -(2^p),(2^p-1)

the new function can be used as a special case for av_clip(), e.g.
av_clip(x, -8192, 8191) can be rewritten as av_clip_intp2(x, 13)

there are ARM instructions, usat and ssat resp., which map nicely to these
functions (see next patch)
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>