User request: oiiotool --zover for z depth composite

New ImageBufAlgo::zover() and exposure as oiiotool --zover : it's just like "over" except the decision about which image is foreground and which is background happens on a per-pixel basis depending on the value of the z channel (the presence of which is required in both input images).

I also decided to relent and simplify the implementation of over() by requiring the input images to have alpha channels and the same number of channels. I know I resisted that months ago, but now I think I was wrong and simpler is better.

This patch also contains a very minor fix to the rla reader... it wasn't labeling anything as "z", so made it do so for the first 32 bit "auxiliary" channel.