Added Tileset.sprite_at ...

[ahnlak]

...to allow drawing of sprites directly from the tileset.

While I could load up the tileset as a separate sprite sheet, given I was overdrawing the tiled map, if felt more logical to be able to directly draw from the tileset itself, by tile ID.

(obviously feel free to delete this PR if if feels like too niche of a requirement - I may be doing something ... weird by needing it!)