Normal Vector Using WorldInverseTranspose

shader里面经常看到normal向量是用WorldInverseTranspose矩阵做变换的，有时候也可以用WorldMatrix变换。

原理：

If your object is only ever rotated and translated, you can transform normals using the top left 3x3 portion of the world matrix.

If it is rotated, translated, and uniform scaled, you can do that same
3x3 matrix transform, but must then re-normalize the output vector to
remove the scaling.

But if the transform includes a non uniform scale, shear, taper, etc,
normals would no longer be correct after just a rotation. To get the
right normals in such situations, you must transform them by the inverse
transpose of the world matrix, then re-normalize the output vector.

The math is derived here: http://explodedbrain.livejournal.com/112906.html

Some shaders do this the complex slow way in order to be as robust as
possible. Others figure that nobody in their right mind would ever use a
non-uniform scale in a game, so they don't bother to support that,
which simplifies the shader and makes rendering more efficient (but can
lead to confusion if anyone ever does use an unsupported transform and
gets bogus lighting results).

总结： 如果object没有涉及到x,y,z方向的不统一缩放，就可以直接用WorldMatrix，否则用WorldInverseTranspose.

详细介绍参考：http://www.arcsynthesis.org/gltut/Illumination/Tut09%20Normal%20Transformation.html