Add initial prototype.

3-mid/opengl/private/gid/gid-decoding_gif.ads

@@ -0,0 +1,22 @@
+private package GID.Decoding_GIF is
+
+  --------------------
+  -- Image decoding --
+  --------------------
+
+  generic
+    type Primary_color_range is mod <>;
+    with procedure Set_X_Y (x, y: Natural);
+    with procedure Put_Pixel (
+      red, green, blue : Primary_color_range;
+      alpha            : Primary_color_range
+    );
+    with procedure Feedback (percents: Natural);
+    mode: Display_mode;
+  --
+  procedure Load (
+    image     : in out Image_descriptor;
+    next_frame:    out Ada.Calendar.Day_Duration
+  );
+
+end GID.Decoding_GIF;