Add custom editor_delay export variable

[Joy-less]

For smaller pixel-art textures (e.g. 32x32) generating the mesh is practically instantaneous, so it can be ideal to reduce the generation delay for the editor.

[98teg]

Could you please add it to the documentation too?

[Joy-less]

Could you please add it to the documentation too?

Sure, done :)

[98teg]

Thanks!