EXPERIMENTAL: Rotating windows.

* Dumb bounding box top_at logic replaced with select buffer
* Select buffer rendered through cairo with AA disabled
  Using rectangles for window shapes - this should easily
  be expandable to 1bpp bitmaps. Currently, the select buffer
  is very inefficient, using twice the space it needs (plus,
  it's double buffered, so in reality, 4x the space needed),
  however, it's also very accurate and fast, and I like that.
* Window rotation is controlled through Ctrl+Shift+{z,x,c} where
  z = rotate 1 degree left
  x = rotate 1 degree right
  c = reset rotation
* Input is remapped based on window rotation, so you *can* use the
  draw app, and it is totally epic.