Add support for SSMF Detail Normal Textures:

Works similarly to the previous SSMF splatting, but instead of sampling a combined texture of 4 different single-channel diffuse detail textures, it samples four separate normal textures.
The textures must contain the normal maps in the RGB channel (standard normal maps), with a diffuse luminance in the alpha  channel.
The system uses the same splatDistrTex as regular SSMF.

Usage:
	Specify splatDetailNormalTex1 .. splatDetailNormalTex4 in the MAP resources section.
	splatDetailTex will be unused if the above are present.

Yes this is DSD: http://beherith.eat-peet.net/stuff/detail_normals.png

![detailnormals](https://cloud.githubusercontent.com/assets/109391/4337155/c5dd5bf6-400c-11e4-9e4b-ca897f30f909.jpg)