直交射影行列を作成します。
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.