Previously, this function would do coordinate calculations in such a
way that (x_dst, y_dst) would only affect the alignment of the source
image, but not of the traps, which would always be considered to be in
absolute destination coordinates. This is unlike the
pixman_image_composite() function which also registers the mask to the
destination.

This patch makes it so that traps are also offset by (x_dst, y_dst).

Also add a comment explaining how this function is supposed to
operate, and update tri-test.c and composite-trap-test.c to deal with
the new semantics.

This program tests whether the new triangle support works.