Creates an orthogonal projection matrix.
The returned matrix is such that views left to right, bottom to top area, with
zNear and zFar depth clipping planes.
If you want to use orthogonal for pixel-correct rendering, it is better to use GL.LoadPixelMatrix,
as that would apply proper half-texel offsets for Direct3D renderer.
See Also: GL.LoadPixelMatrix, GL.LoadProjectionMatrix, GUI.matrix.