Add a checkerboard pattern to the default grid material to make subgrid divisions easier to count

[Calinou]

• Related to Use anisotropic filtering on grid material, set roughness texture #8.

Right now, the default material is just a grid, with no visible subdivisions.

Similar to what I proposed in godotengine/godot-proposals#6138, I suggest adding a subtle checkerboard pattern that tiles twice on each grid square (which means a 4×4 checkerboard pattern is visible on the texture). This would make it easier to count subdivisions when working with smaller grid sizes, as each checkerboard tile would be 0.25 units by default.

I was thinking about making those adjustments myself, but I noticed there are Aseprite source files (and I don't have Aseprite to modify those directly).