Add spin ability to scrolling textures

[DVL-IAC]

Would be cool if scrolling textures had an option to just spin in place instead of only pan options. Would be good for creating clouds that spin slowly in the skybox or a giant portal in the sky effect and probably many other neat effects.

Currently the only ways to do a spinning texture would be to either put it on a brush on a rotate in place keyframe, but keyframes don't work in skyboxes, or create a vbm of a spinning texture, but vbm's don't look great and would need hundreds of frames to make the spin look smooth, which would be a massive file and a pretty time consuming process to make all the frames.

[BATEMAN87]

This feature would be very good for wheels rotating on a keyframed vehicle.