Fix bsp model texture animation

[res2k]

Fix missing texture animation on bsp models (issue #185) by "animating" the frame in load_and_transform_triangle().

Another approach I considered was to go over each bsp model once per frame and animating the material on each primitive - think similar principle as instance_geometry, only for bsp models, and material animation only.

I went for the load_and_transform_triangle() approach mainly because it's simpler.
However, it may have a performance disadvantage, as the "animation" would potentially be done quite a number of times, compared for guaranteed once per frame in the approach above.
(Perhaps it would be worthwhile to implement that as well and check if there's a measurable performance difference.)

[PalmliX]

Thanks for this res2k! I'm experimenting with some very long animations, over 100 frames, so I'll let you know about the performance.