Slow noise previews freeze the editor

[Xrayez]

Module version: 2.0-alpha

Issue description: Currently it might take too long for the noise preview to get generated with lots of visual noise nodes, especially with mapping modes set to SEAMLESS_XY or SEAMLESS_XYZ, which causes the editor to freeze. Some very long noise chains may take up to 10-20 seconds to generate a seamless image.

The solution would be to generate noise previews in a thread instead, possibly with ability to cancel noise preview generation for a particular node.

[Xrayez]

The issue can be partially salvaged by setting lower preview quality that can speed up noise generation as available in 40ee0e9.

Go to Editor -> Editor Settings -> Editors -> Visual Accidental Noise -> preview_quality, and set the value to 0.25 or 0.5, and this going to speed up things by a factor of 4 depending on the mapping modes.