Initial port of zfreeze branch (3.5-1729)

Initial port of original zfreeze branch (3.5-1729) by NeoBrainX into
most recent build of Dolphin.

Makes Rogue Squadron 2 very playable at full speed thanks to recent core
speedups made to Dolphin. Works on DirectX Video plugin only for now.

These are the Game Settings I have for GSWE64.ini under edit config
===============
[Core]
DSPHLE = False
[DSP]
EnableJIT
[Video_Settings]
AspectRatio = 1
FastDepthCalc = False
===============

Enjoy!  and Merry Xmas!!
Just write to constants.zslope directly. There is no need for a temporary storage.