add ffmpeg_duplicate_frames option

This lets the user decide which video problems they would rather see.
In my case a Raspberry Pi 2 with a 640x480 webcam sometimes can keep
up with 10fps so I don't want to set the framerate any lower, but
then sometimes it can't and with duplicate frames it looks like the
video output freezes every second.

conf.c

@@ -668,6 +668,15 @@ config_param config_params[] = {
     print_string
     },
     {
+    "ffmpeg_duplicate_frames",
+    "# True to duplicate frames to achieve \"framerate\" fps, but enough\n"
+    "duplicated frames and the video appears to freeze once a second.",
+    0,
+    CONF_OFFSET(ffmpeg_duplicate_frames),
+    copy_bool,
+    print_bool
+    },
+    {
     "output_debug_pictures",
     "# Output pictures with only the pixels moving object (ghost images) (default: off)",
     0,

conf.h

@@ -30,6 +30,7 @@ struct config {
     int max_changes;
     int threshold_tune;
     const char *output_pictures;
+    int ffmpeg_duplicate_frames;
     int motion_img;
     int emulate_motion;
     int event_gap;

motion.c

@@ -570,8 +570,13 @@ static void process_image_ring(struct context *cnt, unsigned int max_images)
             /* 
              * Check if we must add any "filler" frames into movie to keep up fps 
              * Only if we are recording videos ( ffmpeg or extenal pipe )         
+             * While the overall elapsed time might be correct, if there are
+             * many duplicated frames, say 10 fps, 5 duplicated, the video will
+             * look like it is frozen every second for half a second.
              */
-            if ((cnt->imgs.image_ring[cnt->imgs.image_ring_out].shot == 0) &&
+            if (!cnt->conf.ffmpeg_duplicate_frames) {
+                /* don't duplicate frames */
+            } else if ((cnt->imgs.image_ring[cnt->imgs.image_ring_out].shot == 0) &&
 #ifdef HAVE_FFMPEG
                 (cnt->ffmpeg_output || (cnt->conf.useextpipe && cnt->extpipe))) {
 #else