vp9-svc: Fix to setting frame size for dynamic resize

vp9-svc: Fix to setting frame size for dynamic resize

For svc with dynamic resize (only for single_layer_svc mode),
add flag to indicate resized width/height has already been set,
otherwise on the resized/trigger frame (resize_pending=1), the
wrong resolution may be set if oxcf->width/height is different
than layer width/height in single_layer_svc mode.

Change-Id: I24403ee93fc96b830a9bf7c66d763a48762cdcb4