More big changes.

- Added depth sorting support to atlas, atlas::image, atlas::rect_image,
atlas::image_factory.
- Fixed a bug in video_tile_buf_renderer that caused a crash if the map
went outside of the clip rect.
- Added support for special symbol tiles and factory ownership switching
on images as a mechanism for doing it the fewest texture switch calls
possible.
- Added implicit conversions from vector2i to phoenix::Vector2d.
- Updated pattern buffer renderer to use the system tileset.
- Updated widget system to use the map_metrics struct to pass
information to renderers.