Add tests and border collision
Issue new shape when existing shape hits the bottom of the board
Remove defunct draw_shapes code
Make Game into a GenServer

...and just make the Window render the Game regularly
A little addition to Episode 119
Introduce `board` key to the State struct
Overlay shape onto board for renderer
Draw based on numbers in board data

Previously, the brush used was always for the current shape.  This is
wrong and dumb.  Oops.
Handle collisions and validations

There were a few collision and validation steps that weren't in place.
This fixes that.  Now it's almost a full tetris game, though you can't
yet clear a line.
Remove useless function clause
Add line clearing
Merge remote-tracking branch 'origin/master' into feature/sdl
Draw a rect with SDL...