Add initial prototype.

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

@@ -0,0 +1,36 @@
+with
+     GL.Binding,
+     openGL.Tasks;
+
+package body openGL.Viewport
+is
+   use GL;
+
+
+   function Extent return Extent_2d
+   is
+      use GL.Binding;
+      Extent : array (1 .. 4) of aliased gl.glInt;
+   begin
+      Tasks.check;
+      glGetIntegerv (gl_VIEWPORT,
+                     Extent (1)'unchecked_Access);
+
+      return (Integer (Extent (3)),
+              Integer (Extent (4)));
+   end Extent;
+
+
+
+   procedure Extent_is (Now : in Extent_2d)
+   is
+      use GL.Binding;
+   begin
+      Tasks.check;
+      glViewport (0, 0,
+                  GLint (Now.Width),
+                  GLint (Now.Height));
+   end Extent_is;
+
+
+end openGL.Viewport;