Lazy texture loading

[Per]

I have implemented what I decided to call "lazy texture loading", which basically means that you no longer need to list every texture that you use in the relevant wrf files. When models are loaded, the code will pull in the required textures on demand, both ordinary ones and tcmask textures. It will assume that you want a tcmask texture if the tcmask flag is used in the model. For the actual code, see #2304.

As side-effect consequence of this, however, the code now requires that the texture reference in the model refers to an actual file, not just a shortcut like "page-10.png", which worked previously. Otherwise it will not know what to load when lazy texture loading is used.

For now, this is only implemented in master. It may be backported to 3.0 later when people have had some time to test it.

This for your information.