Support for glfw's CursorDisabled

[jared-nishikawa]

From https://github.com/beta/glfw/blob/master/glfw.go#L2990:

// - CursorDisabled hides and grabs the cursor, providing virtual and unlimited
// cursor movement. This is useful for implementing for example 3D camera
// controls.

I was experimenting with pixelgl's current best option for 3D camera controls, but I was getting unpredictable results on different operating systems (for instance, on a mac resetting the cursor to the center was causing about 1 second of delay).

I added this in pixelgl/window.go:

func (w *Window) DisableCursor(enabled bool) {
    mainthread.Call(func() {
            w.window.SetInputMode(glfw.CursorMode, glfw.CursorDisabled)
    })
}

and disabling the cursor in this way (instead of manually hiding and resetting it) seemed to solve my problems. Poking through the rest of window.go I can tell that this function is not really idiomatic (cf. SetCursorVisible which references a cursorVisible parameter in the Window struct). I'd be happy to make a PR (if indeed this seems like something worth adding), but wanted to talk through how best to fit it in.