Viewport Improvements

Some minor changes which will soon bring major functionality:

1. Viewport now starts centered on a random tile
   1. "Centered" is bracketed by the actual map edges, so the selected tile may not be in the center of the screen
2. Screen Scrolls by half-tile increments
   1. This gives a sense of motion against the "fog"
3. Added "Highlighted" tiles.  Currently this just shows a square sprite, but will eventually be more nuanced.