eglfs: Implement nativeResourceFunctionForContext

Add support for a get_egl_context NativeResourceForContextFunction to
the eglfs platform plugin.

Change-Id: I155952797f340dd00ab9864852add4b44d841042
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
eglfs/imx6: Implement platformDisplay() in hooks

Return the native display created in QEglFSImx6Hooks in
platformDisplay().

Change-Id: I2b28ac66dce12a1054fc2f78dbfad0723f8e5688
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
eglfs: Implement nativeResourceForWindow

Add support to request EglDisplay resource for a window. This is
supported in other platform plugins, like kms and xcb and is required by
wayland-egl based QtWayland compositors.

Change-Id: If3f8c625682915c9671e820974e810763c3c8b66
Reviewed-by: Andy Nichols <andy.nichols@digia.com>