Fixed bounding box problems in readback/binaryswap

Back in June I mentioned that the bounding box code in the readback 
and binaryswap SPUs was broken.  Basically, only the last 
user-specified bounding box was being used to compute the screen-space 
bounds of the rendering (all previous ones were effectively ignored).

Now there's a bboxUnion field in the WindowInfo struct that keeps 
track of the window-space union of all bounding boxes specified during 
a frame.  That bounding box is then used during compositing.

I also updated the psubmit.c and spheres.c demos to specify bounding 
boxes when the -bbox option is given.  Things appear to be working 
correctly now.

-Brian


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