Add initial prototype.

3-mid/opengl/source/lean/opengl-viewport.ads

@@ -0,0 +1,10 @@
+package openGL.Viewport
+--
+--  Models an opendGL viewport.
+--
+is
+
+   function  Extent       return Extent_2d;
+   procedure Extent_is (Now : in Extent_2d);
+
+end openGL.Viewport;