Add initial prototype.

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

@@ -0,0 +1,18 @@
+private package GID.Decoding_BMP 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);
+  --
+  procedure Load (image: in out Image_descriptor);
+
+end GID.Decoding_BMP;