imag.py: Optimise the hell out of it!

Forget using a PIL file decoder plugin - that restricts me far too much
to write a semi-fast implementation since I'm not writing it in C.

Instead, use the multiprocessing module to construct image strips in
parallel, then stitch them back together at the end. Also try to avoid
rearranging data within each 4x4 block which was killing my performance
before.

With this, my dual-core i5 can load a 1024x1024 image in under a second.

It's still not going to be anywhere near as fast as an optimal C
implementation using SIMD type instructions, let alone a GPU
implementation, but it will suffice for my purposes.