scan: enable HBTF_NO_IDR when not enough previews found

Setting this flag signals to libav to not wait for IDR or recovery
points before returning frames to us.  Some videos have neither IDRs or
recovery points, so this fixes transcoding such video.

Fixes #456

libhb/scan.c

@@ -223,6 +223,10 @@ static void ScanFunc( void * _data )
         npreviews = DecodePreviews( data, title, 1 );
         if (npreviews < 2)
         {
+            // Try harder to get some valid frames
+            // Allow libav to return "corrupt" frames
+            hb_log("scan: Too few previews (%d), trying harder", npreviews);
+            title->flags |= HBTF_NO_IDR;
             npreviews = DecodePreviews( data, title, 0 );
         }
         if (npreviews == 0)