vnc: tight: don't forget the third color

While couting color, if the third color was only present one
time it wasn't added to the palette.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

vnc-encoding-tight.c

@@ -177,6 +177,7 @@ static int tight_palette_insert(QDict *palette, uint32_t rgb, int bpp, int max)
         *palette = qdict_new();                                         \
         tight_palette_insert(*palette, c0, bpp, max);                   \
         tight_palette_insert(*palette, c1, bpp, max);                   \
+        tight_palette_insert(*palette, ci, bpp, max);                   \
                                                                         \
         for (i++; i < count; i++) {                                     \
             if (data[i] == ci) {                                        \