Consider changing all coordinates to (f32, f32)

It is often desirable to track motion as sub-pixel values. Additionally, cursor events will be (f32, f32) and may be negative, if the cursor is out of the window.

Additionally, we could change the render function to take a PixelBuffer, and support a `pixel_at(f32, f32)` function that returns an Option, which would be useful for rendering objects that may be partially off screen.