complete tile drawing support in Data::StarCraft::Tileset

Read minitile data (including full bitmaps) and palette.

Several new methods to get tile information:
- tile gives general details, including an image of one pixel per
  minitile.
- tileavg aggregates all minitile data to just one total per tile,
  including the average color of all minitiles.
- sprite gives the full bitmap of any tile.

Replaces manual guesswork in Data::StarCraft::Map and conversion in
scmap.