Fix Gtk3Backend window height resizing

[stackotter]

To reproduce this issue, create a window with some content that expands to fill the full height of the window (e.g. Color.red), and give the window .contentMinSize resizability with the .windowResizability modifier. You'll find that you can freely resize the width of the window, but can always increase the height of the window, but can't decrease it.

This is likely an issue with how we're decoupling the window's size from the size of the root view, which involves our purpose-made CustomRootWidget widget.