Added new coordinate type: Screen.

For further map object units work, there needs to be a global distinction between map coordinates and screen coordinates, so this pull does the following:

- The old pixel coordinate type is replaced with the screen coordinate type, and all calls are modified to match.
- This is replaced with a new pixel coordinate type, which represents pixel coordinates in map space.
- Adds these functions to MapRenderer: pixelToScreenCoords, screenToPixelCoords, tileToScreenCoords, screenToTileCoords.