Support Image Fill Tiling

[mad-moo]

Add a new fill mode for image fills, that makes them tile. This is often useful for material-y patterns, such as dots, squiggly separators and such.

I'd propose the following changes:

• Add a new fill_mode to rio.ImageFill: "tile"
• Add another parameter: tile_size: tuple[float, float] for controlling how large each individual tile is. This parameter is ignored in other fill modes (add that to the docs).

[mad-moo]

I implemented this yesterday, but still have to test it properly.