niimaqsrc: add avoid copy mode

Unfortunately IMAQ only lets you examine one buffer at a time, and elements
like queue store up multiple buffers, so avoid-copy=TRUE can only be used in
pipelines without queues or other such elements.

Also, make property strings static.

sys/niimaq/gstniimaq.h

@@ -52,6 +52,7 @@ struct _GstNiImaqSrc {
   /* properties */
   gchar *interface_name;
   gint bufsize;
+  gboolean avoid_copy;
 
   /* image info */
   GstVideoFormat format;