Add basic autotiling with sand pit

[Sdc97]

Issue

Fixes #18

Notes

• Added terrains (autotile) to the world tilemap
  • Added sand, dark sand, and grass layers of tilemap
  • Created tile mappings using corners only
• Updated world tilemap to extend right into a sandy area

Note: There is a small bug where edges of grass tiles may map to a dirt edge on the end of the map. A separate issue for this will be opened after this PR

Testing

• New area showing the tilemap terrains

[choduh]

good :)