Refactored ProjectionStack class

Renamed ProjectionStack -> ViewportHelper.
This name better reflects the methods contained within the class.

Made class static, removed dependence on immediate mode OpenGL

Split GetCurrentOrthogProjection method into IsOrthographicProjection and GetViewportFromOrthographicProjection

Removed TransformViewport, and made Viewport class use floats (vs. ints)
Note: Had to update viewport retrieval code, now uses an intermediate array of ints

Refactored OrthogonalTransform method in QFont.cs to use the new ViewportHelper methods

Cleaned up Begin and End methods in QFont.cs as a result.
The End method is possibly not needed anymore, unless there is cleanup that needs doing.