2x faster quant. 2% overall.

2x faster quant. 2% overall.
side effects:
not bit-identical to the previous algorithm.
while the new algorithm covers a wider range of cqms than the previous one did,
I couldn't find a good way to fallback to a general version for the extreme
cqms. so now it refuses to encode extreme cqms instead of just being slower.
lays a framework for custom deadzone matrices, though I didn't add an api.

git-svn-id: svn://svn.videolan.org/x264/trunk@642 df754926-b1dd-0310-bc7b-ec298dee348c