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/image_decoder.h

@@ -32,6 +32,7 @@ enum image_type {
 #ifdef HAVE_LCD_COLOR
     IMAGE_PPM,
 #endif
+    IMAGE_GIF,
     MAX_IMAGE_TYPES
 };