Added the helper functions needed in the canvas to implement the enemy class.
For now, the KOBOLD.visible has been set to true, because the Bresenham lines
don't pass over the top_layer.  Some thought will have to go into how this should
be fixed.  Really, what should happen is the canvas shouldn't be pointers, and everything
else should do it's calculations from the buffer.  Then, the canvas and top_layer could
just be smashed together.
Merge branch 'master' of https://github.com/schiem/roguelike