Unset the current gl window when deleted

This does in the QSGGuiThreadRenderLoop what 403045a did for the
QSGThreadedRenderLoop. Fix a memory corruption in Mesa's dri2 EGL
when setting a window current after deleting the previous current
one.

Change-Id: I8df0bacdf12f71e969a9ea39842a402c9a2a303f
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>