This drops the "feature" to load level 0 from the custom texture
and all other levels from the native one if the size matches.
But in my opinion, when a custom texture only provide one level,
no more should be used at all.

EFBToScaledXf and EFBScaledYf return a float, so the cast isn't needed here.

Had to re-do *ShaderManager so they saved their constant arrays
instead of completly rebuilding them on restore state.

- 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

OpenGL requires the y coordinates to be flipped.

Also refactored PixelGen code to remove duplicate code.

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.