Texture splatting preview

I've made some more progress on texture splatting, some of which is now described on the algorithms page (with pretty pictures). With regards to this feature, I still have to:

• Export alpha maps as mosaic images (i.e. split images into tiles).
• Finish the implementation of the user-interface.

Unless there are any objections, I plan to remove the old 'export texture list' option, as it is rendered obsolete by proper texture splatting support. This option was a cheap-n-nasty way of getting the alpha-maps for run-time blending, but required the use of the attributes map file rather than a set of image files.