add vips_sum()

sum an array of images
Merge branch 'master' into hough
stuff
basic hough transform seems to work
comment improvements
Merge branch 'master' into hough
started splitting hough.c up
split hough to a base class and a line detector
use a LUT for hough sin/cos
small hough cleanups
add vips_draw_add()

and start vips_hough_circle()
start to extend draw_image
put gtk-doc.make back

some problems in gtkdocize make it hard to live without
add "mode" to vips_draw_image()
vips_hough uses vips_draw_image() to accumulate

rather than vips_sum() ... 2x faster
add a direct path for draw_mask

and get draw_circle ready for draw-along-circle
clean up draw_circle

we now have a simple iterator
start another draw cleanup
finish revising draw_line
vips_hough_circle() added