New material properties

Adds following material flags
- unlit - no lighting, diffuse value can still be used to tint the result
- two_sided - self-explanatory, however normals on the other side are not automatically inversed (no good way to do this) so the usefulness is limited
- alpha_test - pixels with alpha value < 0.5 are discarded, this is good for fences and such. It produces sharp edges but the geometry does not need to be sorted.

Luomu/newmodels/test_matprops can be used to test.