Pass image buffers forward instead of backward

World render will pass its image to region render, which will pass the
image to chunk render.

Previously the chunk would pass an image back to the region, which would
copy it into the region image and pass this back to the world.

This takes out a lot of calls to memcpy() and combine_alpha().

It also removes some faint blending artifacts that were showing up in
isometric renders. The change is visible in the example isometric images
(look at the water).

Also removed a memory leak where ndata[] wasn't being freed.