Instead we keep the loaded vertices in CPU memory.

Also:
 * Implement support for per-vertex PosMatrixIndex
 * Only update zslope constant once when zfreeze is activated.
 * Added a bunch of comments.

- Calculate ZSlope every flush but only set PixelShader Constant on Reset Buffer when zfreeze
- Fixed another Pixel Shader bug in D3D that was giving me grief

Results are still not correct, but things are getting closer.

 * Don't cull CULLALL primitives so early so they can be used as reference
        planes.
 * Convert CalculateZSlope to screenspace coordinates.
 * Convert Pixelshader to screenspace coordinates (instead of worldspace
        xy coordinates, which is totally wrong)
 * Divide depth by 2^24 instead of clamping to 0.0-1.0 as was done
        before.

Progress:
 * Rouge Squadron 2/3 appear correct in game (videos in rs2 save file
         selection are missing)
 * Shadows draw 100% correctly in NHL 2003.
 * Mario golf menu renders correctly.
 * NFS: HP2, shadows sometimes render on top of car or below the road.
 * Mario Tennis, courts and shadows render correctly, but at wrong depth
 * Blood Omen 2, doesn't work.

Based on the feedback from pull request #1767 I have put in most of
degasus's suggestions in here now.

I think we have a real winner here as moving the code to
VertexManagerBase for a function has allowed OGL to utilize zfreeze now
:)

Correct use of the vertex pointer has also corrected most of the issue
found in pull request #1767 that JMC47 stated.  Which also for me now
has Mario Tennis working with no polygon spikes on the characters
anymore!  Shadows are still an issue and probably in the other games
with shadow problems.  Rebel Strike also seems better but random skybox
glitches can show up.

Initial port of original zfreeze branch (3.5-1729) by neobrain 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.

Enjoy!  and Merry Xmas!!