Support two-channel normal maps

Currently normal maps are required to have at least three channels -- RGB for the normal, alpha for the optional height. Now that BC5-compressed DDS file support has landed in OSGoS, it's natural to entertain the thought of supporting two-channel, or red-green normal maps, which are currently unusable. For them, the Z component of the normal needs to be reconstructed in the shader.

Come Fallout 4, Bethesda elected to switch to one of the compressed formats, BC5 for normal maps in their art pipeline, for a good reason: it is, in fact, the most optimal format for storing normal maps, taking up the same amount of memory as DXT5 yet coming with a dramatically lower quality loss. We can follow their example.

The proposal is to auto-detect if the image used for the normal map uses one of the applicable pixel formats and set a Z component reconstruction flag based on that. While Creation Engine most likely doesn't target more than a couple normal map pixel formats, it is unlikely mods that could rely on that exist (or at least, I'm very curious to figure if they do). If it's deemed necessary, NIF files intended to be used with 2015+ Creation Engine could force the use of that flag in the future.

If the used OpenSceneGraph release or hardware doesn't support RGTC compression, the use of an uncompressed red-green format would still allow to save a whole byte of memory per texel.

Naturally two-channel normal maps cannot support parallax. The most practical option is to use uncompressed RGBA normal maps for normal maps with parallax. Slightly less practical options are to pray that BPTC support becomes a thing in OSG or to pray that it just works in vsgXchange. Another suboptimal option is to use the recently added diffuse map alpha parallax support, but it requires changes to the NIF files (and their use to begin with) and isn't suitable for terrain or transparent objects.