At the moment GL/VK content can only be presented in top-level windows, since child windows are not backed by Wayland surfaces. This MR adds support for such scenarios, in a few gradual steps:
1. Create Wayland (sub)surfaces for all child windows, anchoring them to their parent surface, which may also be a child window surface (i.e., we support GL/VK in nested child windows). This approach works, but it pollutes the compositor with mostly unused, and possibly nested (sub)surfaces. We will deal with this later in the MR. 2. Ensure that the child window (sub)surfaces are properly updated and reconfigured, and support WS_POPUP <-> WS_CHILD style changes (reparenting etc). 3. In the last commit, improve efficiency by creating (sub)surfaces only for the child windows needed by GL/VK, and anchor them directly to the parent toplevel. This removes (sub)surface bloat and unnecessary nesting, the trade-off being some extra complexity when dealing with updates.
Note that this MR doesn't clip GL/VK child window contents at the moment.
The subsurface mechanism introduced in this MR could also handle other kinds of windows in the future, for example display and properly position transient windows, menus etc.