Alternate texture sets for the individual source channels

[harag-on-steam]

According to Keen's texture converter tool SE uses this set of input channels:

• albedo (RGB, sRGB) as defined by PBR
• metal (single, sRGB) as defined by PBR
• gloss, (single, linear) the inverse of roughness, also PBR
• normal (RGB, linear) standard normal map
• ao (single, sRGB) ambient occlusion
• emissive (single, sRGB) emissive intensity, color is taken from albedo
• dirt (single, sRGB) unknown use, defaults to full intensity
• colormask (single, sRGB) masks recolorable parts of a texture
• alphamask (single, linear) masks opaque parts of a texture, not all intensity levels are used in all situations. Decals do full blending but by default materials are either completely opaque > 0.5 or completely transparent < 0.5. For decals the other channel also get alpha-premultiplied by this channels, for default materials this doesn't happen.

The channels are packed into .dds textures (RGBA):

• cm is [albedo, metal] (BC7, sRGB)
• ng is [normal, gloss] (BC7, linear)
• add is [ao, emissive, dirt, colormask] (BC7, sRGB)
• alphamask (BC4, linear)

The Dx11 shader node the addon provides already has inputs for the original channels. Allow to switch the texture-slots of a material into a mode that has the individual channels as file slots instead of the combined final textures.

In this mode the addon expects the source images in a different format than .dds and derives final texture-filenames from those. A decision is needed how to handle situations like basename1_color, basename2_metal ⇒ basename?_cm should be handled.

An added bonus would be to initiate the texture conversion directly from Blender instead of going through Keen's texture converter.