Overlay plane support in atomic eglfs_kms

• Allow mapping a QWindow to an overlay plane. This breaks certain assumptions in quite a few places (also outside eglfs backend), since one window per screen is not true anymore). The current, simple approach of 1. screen owns a fullscreen gbm_surface 2. which is the window surface for the (fullscreen) window on that screen 3. and gets used in drmModeSetCrtc and drmModePageFlip is also not quite good enough in the new setup.
• QScreen remains associated with the entire output. It is QWindow (and possibly multiple ones) which we want to map to the primary and overlay plane(s).
• The plane geometry could be picked up from the window
• Others to be specified by the extra API, to be synced with Renesas/VSP efforts

Changed this to be dependent on atomic modesetting and nuclear flips. Initial proof of concept can be done with the legacy API, though.

References:
https://en.wikipedia.org/wiki/Direct_Rendering_Manager#Atomic_Display
https://lwn.net/Articles/653071/
https://doc.qt.io/qt-5/embedded-linux.html#advanced-eglfs-kms-features