int im_ifthenelse(c, a, b, out) IMAGE *c, *a, *b, *out;
int im_blend(c, a, b, out) IMAGE *c, *a, *b, *out;
builds an output image which uses some pels from a and some from b: if the
conditional image c is non-zero at that point, the pel comes from a; if it
is zero, the pel comes from b.
The conditional image c can have either 1 band, in which case entire pels
come either from a or b, or n bands, where n is the number of bands in both a
and b, in which case individual band elements are chosen from a and b.
Images a and b must match in size, type and number of bands.
works just as im_ifthenelse(3), except that instead of selecting between a and
b, values in the condition image are used to softly blend between the two. 255
means a only, 0 means b only, 128 means 50:50.