Add initial prototype.

3-mid/opengl/source/platform/glx/private/opengl-surface-privvy.adb

@@ -0,0 +1,10 @@
+package body openGL.Surface.privvy
+is
+
+   function to_GLX (Self : in Surface.item'Class) return glx.Drawable
+   is
+   begin
+      return Self.glx_Surface;
+   end to_GLX;
+
+end openGL.Surface.privvy;