Add methods to get the position of a window's client area, relative to the desktop

[Osspial]

This differs from the current get_position, in that the current method gets the window's top-left position, including window decorations. This adds a get_inner_position method, which gets the client area's top-left position, not including decorations, relative to the desktop.

[francesca64]

For me under Xfwm4, get_position and get_inner_position return the same values. However, since that's the result of more fundamental issues with geometry calculation in the X11 backend, I think it's fine for now.

[mitchmindtree]

I just tested this on macos 10.12.4 by printing the result of window.get_position() and widow.get_inner_position() within the event loop of the window.rs example and watching the values as I move the window around. Both methods produce the same result which seems to be the "outer" position (top left of the decorations).

Not sure if the get_inner_position mac implementation should be fixed or not before this is merged. Perhaps at least it could be changed to unimplemented!() in the meantime to avoid weird unexpected behaviour issues?

[francesca64]

Okay, with #438 merged, this works on X11 now.

@mitchmindtree are you sure they were both outer position? I changed the relevant code (and badgered one of my friends into testing it), and it seems that the existing get_position implementation was returning the content area position (judging by both the values returned and the use of contentRectForFrameRect). So, I changed get_inner_position to be identical to the current get_position, and changed get_position to just use the frame rect instead of requesting the content rect.

(Also, it seems that on macOS, set_position sets the content area position. On X11, we set the outer window position, and I'm not sure about the other backends, but that's a concern for a separate issue.)

EDIT: the set_position implementation appears to be informed by that incorrect get_position implementation, and it would be easy to fix: https://github.com/tomaka/winit/blob/591e0d9b8e9f93675c25c70acea846a5d50dae46/src/platform/macos/window.rs#L543-L547

[francesca64]

Okay, set_position works the same way on Windows as it does on X11, and the docs specify that set_position uses the same coordinates as get_position. I've corrected the implementation of set_position on macOS accordingly.

[francesca64]

@RobSaunders do you think you could take a look at this?

[francesca64]

Alright, after getting a chance to test this on macOS hands-on, I've confirmed that the new implementation is correct, and consistent with Windows and X11.