Fix compatibility breakage from adding NoiseTexture3D

[clayjohn]

Follow up to #76486

#76486 broke compatibility by introducing a new parameter to Noise.get_image() and Noise.get_seamless_image(). The new parameter was called p_depth even though the function did not return a 3D image (i.e. an array of Images), so it really represented a z offset.

This PR introduces a get_image_3d() for use with 3D textures and it abstracts the old get_image() into an internal function _get_image() which handles both 2D and 3D.

This PR also:

1. Fixes a case of calling wait_on_thread on a thread that hasn't been started
2. Optimizes the generation of normalized noise textures (the speedup is about 33% even for regular 2D textures, it is significantly more for 3D)

Finally, I have left the seamless generation algorithm the same. Although I suspect it can be generalized to the 3D case a bit more and optimized.

[akien-mga]

Didn't do an in depth review, but the overall changes seem sensible, and undoing the compat breakage is great.

[Lasuch69]

This looks awesome! I wanted to change as little as possible in the current implementation, and now I see that it was unnecessary.

[Calinou]

Tested locally on the MRP linked in #76486, it works.

Should be good to merge after applying suggestions.

[clayjohn]

Should be good to merge after applying suggestions.

Done!

[Geometror]

Looks great! Unfortunately I haven't had the time to review the original PR, but this definitely looks like a sensible fixup/cleanup. Tested it and it looks like everything is still working fine.

[Zireael07]

note that "Depth" and the like are common names for parameters involving z axis

[mhilbrunner]

Thanks for fixing this up! 🎉