Add material for generated SpriteMesh

[Ochmar]

I can't find any specific reason why don't we update Mesh with Material.
It makes it easier to preview and save.

[98teg]

Hi! Thank you so much for taking the time. I'm not sure about this change though. In the case of an animation, every mesh shares the same material. By assigning it to each mesh, we would give the impression that the material of each mesh contains just a single frame of animation, which may cause some confusion. I would leave this open to gather more feedback.

[Ochmar]

Agreed, I did not think about animation.
I was thinking more of generating tons of SpriteMeshes that are static (not animated); hence pull request.
Do you think it'll make more sense to only use it with checkbox as a "setting"?
Something like "Add material to each mesh" (default false).
Or it does not make sense at all in this specific addon?

[98teg]

It would be cool if each material contained only one frame, but that would require changing the UVs of the generated meshes. I'm not planning on adding this right now.