Mouse coordinates are now right-side up.  Moved some of the OpenGL code
into Slick code.
Tiles are now directly associated with their tileset.  This will allow
different tiles to choose what tileset they will draw their graphics
from.
Various minor performance improvements.
Frame xml now accepts character constants in addition to tile indices.
Created the FrameStack for layering multiple tiles into one object.
Created the turtle and giraffe tiles.