FIX: Tests pass under windows

Finally got a decent Windows rig to test on. This gets tests passing, at least for the backends I have on my Windows Python3.4 install (`pyqt4`, `pyglet`, `glfw`).

Ready for review/merge.
Maybe it's just my machine, but it refused to make a window that small for some reason -- one of the shapes (either canvas.size or the screenshot size, can't remember which) ended up being (100, 248) or something. This was a simple workaround for now.