im_makerw(3) attempts to make image writeable, ready for an in-place operation.
Its actions are just as those for im_incheck(3), except that descriptors which
after im_incheck(3) end up as IM_MMAPIN descriptors, magically become
IM_MMAPINRW descriptors.
Extreme caution is urged! You can permanently damage valuable data with this
call.
RETURN VALUE
The function returns 0 on success and -1 on error.