Fixed (maybe) bottom blending mode again.

This fixes the transparency problem introduced
by rev 3103 to DBZ without breaking the Kuro no Danshyou
title screen.

I also tried to make the code a bit a more readable.

git-svn-id: svn+ssh://svn.code.sf.net/p/yabause/code/trunk@3197 d7be59e3-070e-4b9a-85a1-4646befae34d