monoscope: Fixate pixel-aspect-ratio too and make sure the final caps are completely fixated

Otherwise e.g. this fails with assertions:
gst-launch-1.0 audiotestsrc ! audioconvert ! monoscope ! videoconvert ! \
    videoscale ! video/x-raw,width=800,height=600 ! ximagesink
diff --git a/gst/monoscope/gstmonoscope.c b/gst/monoscope/gstmonoscope.c
index e06196b..fa0d541 100644
--- a/gst/monoscope/gstmonoscope.c
+++ b/gst/monoscope/gstmonoscope.c
@@ -255,6 +255,9 @@
   gst_structure_fixate_field_nearest_int (structure, "width", 320);
   gst_structure_fixate_field_nearest_int (structure, "height", 240);
   gst_structure_fixate_field_nearest_fraction (structure, "framerate", 25, 1);
+  gst_structure_fixate_field_nearest_fraction (structure, "pixel-aspect-ratio",
+      1, 1);
+  target = gst_caps_fixate (target);
 
   gst_monoscope_src_setcaps (monoscope, target);