Fix x11 Window::get_position wrong values

[jwilm]

Some window managers like i3wm will actually nest application windows
(like those opened by winit) within other windows to, for example, add
decorations. Initially when debugging this method on i3, the x and y
positions were always returned as "2".

The solution that other xlib abstractions use is to climb up the window
hierarchy until just below the root window, and that window must be used
to determine the appropriate position.

This patch doesn't take into account borders or the window's offset
within its parent, but it's much more usable than the original
implementation on certain WMs.

[tomaka]

Don't we require the parent window at initialization? I feel like this new function is a bit useless?

[jwilm]

@tomaka get_position always returns (2, 2) on my system with the previous implementation. Do you have another idea how we might address that?

[tomaka]

Sorry, I was thinking about the get_window_parent function which is maybe unnecessary? I'm on mobile so I can't really check the code at the moment.

[francesca64]

@tomaka That function is vital for the new logic implemented here, as it's used to climb the window hierarchy in a loop.

[francesca64]

The code looks perfect.

In testing this, get_position used to always return (1, 28) for me, whereas now it returns accurate values, so this is a big improvement.