2gigapix image fixes

If we're doing very large images, where xsize * ysize overflows an int32, we need to have anything that deals with the number of pixels be a py_ssize_t. 

Quantize.c is still loaded with 32 bit sizes, and it's a little more complicated than just the cases in _imaging.c. But I'm pretty sure of these fixes. 

Note that the test isn't run by default, since it requires a reasonably beefy machine and takes ~ 1 minute to run. You have to run it manually. 

Fixes #436