backend-vnc: destroy renderbuffers before renderer output state

Reorder pixman renderer output state and renderbuffer removal to make sure the pixman renderbuffer list is empty when the output state is destroyed.

This fixes an assertion during shutdown:

weston: ../libweston/pixman-renderer.c:1131: pixman_renderer_output_destroy: Assertion `wl_list_empty(&po->renderbuffer_list)' failed.