bmdview generates luminance DDS textures even though Blender does not support them

[julmb]

The supplied bmdview.exe defaults to DDS instead of TGA. If a model contains a luminance-only texture, it gets exported as a luminance-only DDS which seems to be unsupported by many tools, including Blender itself, so they are broken after importing the model using blemd.

The luminance-only textures work fine when using TGA instead of DDS (after fixing issue #11).