imageviewer: gif viewer based on giflib-5.0.2

This adds ability to view gif images in rockbox.
Works both on color and gray/monochrome targets (greylib).
Aspect correction is supported as well.

Limitations:
- animated gifs are restricted to 32 frames
- animated gifs loop always (loopcount is ignored)
- plain text extension is not supported
- animated gifs with interframe delay = 0 are treated as still
  images (web browsers usually treat delay 0 as 100ms to prevent
  exhaustive CPU load by such images)

Change-Id: I61501f801ddcd403410e38d83e6bddc9883e7ede

apps/plugins/imageviewer/bmp/bmp.c

@@ -230,8 +230,9 @@ static int load_image(char *filename, struct image_info *info,
     return PLUGIN_OK;
 }
 
-static int get_image(struct image_info *info, int ds)
+static int get_image(struct image_info *info, int frame, int ds)
 {
+    (void)frame;
     struct t_disp* p_disp = &disp[ds]; /* short cut */
 
     info->width = bmp.width/ds;