libgui|GLState: Setting a normalized viewport

A normalized viewport is useful when the actual root view is larger
or smaller than the render target. A normalized viewport is relative
to the render target size.

doomsday/libgui/include/de/gui/glstate.h

@@ -110,6 +110,13 @@ class LIBGUI_PUBLIC GLState
     GLState &setTarget(GLTarget &target);
     GLState &setDefaultTarget();
     GLState &setViewport(Rectangleui const &viewportRect);
+
+    /**
+     * Sets a viewport that is normalized within the current render target.
+     * @param normViewportRect  Normalized viewport rectangle.
+     */
+    GLState &setNormalizedViewport(Rectanglef const &normViewportRect);
+
     GLState &setScissor(Rectanglei const &scissorRect);
     GLState &setScissor(Rectangleui const &scissorRect);
 

doomsday/libgui/src/glstate.cpp

@@ -384,6 +384,16 @@ GLState &GLState::setViewport(Rectangleui const &viewportRect)
     return *this;
 }
 
+GLState &GLState::setNormalizedViewport(Rectanglef const &normViewportRect)
+{
+    GLTarget::Size const size = target().size();
+    Rectangleui vp(Vector2ui(normViewportRect.left() * size.x,
+                             normViewportRect.top()  * size.y),
+                   Vector2ui(std::ceil(normViewportRect.right()  * size.x),
+                             std::ceil(normViewportRect.bottom() * size.y)));
+    return setViewport(vp);
+}
+
 GLState &GLState::setScissor(Rectanglei const &scissorRect)
 {
     return setScissor(scissorRect.toRectangleui());