libvpx/WebM decoder: lower priority below avcodec

Those people not afraid to use libavcodec should benefit from a faster decoder
http://x264dev.multimedia.cx/archives/499

modules/codec/vpx.c

@@ -47,7 +47,7 @@ static void Close(vlc_object_t *);
 vlc_module_begin ()
     set_shortname("vpx")
     set_description(N_("WebM video decoder"))
-    set_capability("decoder", 80)
+    set_capability("decoder", 60)
     set_callbacks(Open, Close)
     set_category(CAT_INPUT)
     set_subcategory(SUBCAT_INPUT_VCODEC)