Switch to sdl2

Switch to using [go-sdl2](https://github.com/veandco/go-sdl2). The
library seems to be slightly more idiomatic.
Make a window

Create a window with static size (640, 480)
Destroy window at exit

Previously we were calling `sdl.Quit()` but not `window.Destroy()`.
In practice, this shouldn't make much difference but better safe than
sorry.
Add basic event loop

Instead of just quitting as soon as the window is created, we now check
for an sdl ExitEvent and loop until we receive one. Now you can see the
window render before the program exits. Hooray!
Remove superfluous init

So far, creating a window works without the call to init. This will
remove the call until I find a reason that I actually need to have it.
Create a renderer

Even though we're not using it yet, this creates a renderer.