GBM backend for eglfs does not support recreating windows for different screens

Changing the QScreen for a QWindow after create() results in destroying and recreating the QPlatformWindow. The eglfs_kms backend does not seem to support this case. Once an initial association is made between a screen and a window, no other windows can be associated with that screen, even after the first window is destroyed.

The EGLDevice backend reportedly does not suffer from this. (then again the logic is quite different internally, so such differences are not that surprising. nonetheless, the behavior should be identical as much as possible.)