Pass the full image flags to iterators

Pass the full image flags to iterators

When pixman_image_composite32() is called some flags are computed that
indicate various things about the composite operation that can't be
deduced from the image flags themselves. These additional flags are
not currently available to iterators. All they can do is read the
image flags in image->common.flags.

Fix that by passing the info->{src, mask, dest}_flags on to the
iterator initialization and store the flags in the iter struct as
"image_flags". At the same time rename the *iterator* flags variable
to "iter_flags" to avoid confusion.