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.

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

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.

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!!

The other two components need not be valid upon write, hence loading them results in glitches.

Fixes issue 6783.