text: Title: Support Image Fill Tiling
  Body: 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).