Allow Client to disable the rendering of DecorationShadow

For maximized and fullscreen windows we don't want the shadow to be
renderer at all.

client.cpp

@@ -736,6 +736,11 @@ void Client::checkNoBorder()
     setNoBorder(app_noborder);
 }
 
+bool Client::wantsShadowToBeRendered() const
+{
+    return !isFullScreen() && maximizeMode() != KDecorationDefines::MaximizeFull;
+}
+
 void Client::updateShape()
 {
     if (shape()) {

client.h

@@ -605,6 +605,7 @@ public:
 
     bool decorationHasAlpha() const;
     bool isClientSideDecorated() const;
+    bool wantsShadowToBeRendered() const override;
 
     Position titlebarPosition() const;
 

scene.cpp

@@ -826,7 +826,7 @@ WindowQuadList Scene::Window::buildQuads(bool force) const
         }
 
     }
-    if (m_shadow) {
+    if (m_shadow && toplevel->wantsShadowToBeRendered()) {
         ret << m_shadow->shadowQuads();
     }
     effects->buildQuads(toplevel->effectWindow(), ret);

toplevel.cpp

@@ -347,6 +347,11 @@ const Shadow *Toplevel::shadow() const
     }
 }
 
+bool Toplevel::wantsShadowToBeRendered() const
+{
+    return true;
+}
+
 void Toplevel::getWmOpaqueRegion()
 {
     const int length=32768;

toplevel.h

@@ -301,6 +301,11 @@ public:
      * Call this method when the Property changes or Compositing is started.
      **/
     void getShadow();
+    /**
+     * Whether the Toplevel currently wants the shadow to be rendered. Default
+     * implementation always returns @c true.
+     **/
+    virtual bool wantsShadowToBeRendered() const;
 
     /**
      * This method returns the area that the Toplevel window reports to be opaque.