Support 8bpp textures
When textures.xml specifies format="alpha", the input is expected to be an
8-bit greyscale PNG, and the output will be an 8-bit uncompressed
alpha-only DDS.
Add format override to CTextureProperties, to select between e.g. GL_ALPHA
and GL_LUMINANCE for 8-bit textures.
This is needed so fonts can use the new texture system.