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/gif/gif_hash.h

@@ -0,0 +1,39 @@
+/******************************************************************************
+
+gif_hash.h - magfic constants and declarations for GIF LZW
+
+******************************************************************************/
+
+#ifndef _GIF_HASH_H_
+#define _GIF_HASH_H_
+
+//#include <unistd.h>
+#include <stdint.h>
+
+#define HT_SIZE			8192	   /* 12bits = 4096 or twice as big! */
+#define HT_KEY_MASK		0x1FFF			      /* 13bits keys */
+#define HT_KEY_NUM_BITS		13			      /* 13bits keys */
+#define HT_MAX_KEY		8191	/* 13bits - 1, maximal code possible */
+#define HT_MAX_CODE		4095	/* Biggest code possible in 12 bits. */
+
+/* The 32 bits of the long are divided into two parts for the key & code:   */
+/* 1. The code is 12 bits as our compression algorithm is limited to 12bits */
+/* 2. The key is 12 bits Prefix code + 8 bit new char or 20 bits.	    */
+/* The key is the upper 20 bits.  The code is the lower 12. */
+#define HT_GET_KEY(l)	(l >> 12)
+#define HT_GET_CODE(l)	(l & 0x0FFF)
+#define HT_PUT_KEY(l)	(l << 12)
+#define HT_PUT_CODE(l)	(l & 0x0FFF)
+
+typedef struct GifHashTableType {
+    uint32_t HTable[HT_SIZE];
+} GifHashTableType;
+
+GifHashTableType *_InitHashTable(void);
+void _ClearHashTable(GifHashTableType *HashTable);
+void _InsertHashTable(GifHashTableType *HashTable, uint32_t Key, int Code);
+int _ExistsHashTable(GifHashTableType *HashTable, uint32_t Key);
+
+#endif /* _GIF_HASH_H_ */
+
+/* end */