Fix: Texture Error at Trapezoids, ShapeBoxes etc.

Seems the qValue calculation is not correct. This causes texture errors
when having a part which is getting smaller on one side or when using a
shader mod.
Switched back to normal texture rendering as before, the texture
tansformation is no not as good as it should be again.
Merge pull request #355 from Minecraft-SMP/TextureFix

Fix: Texture Error at Trapezoids, ShapeBoxes etc.