Introduce capabilities for renderers (now only symbol levels capability)

src/core/qgsvectorlayer.cpp

@@ -987,7 +987,8 @@ bool QgsVectorLayer::draw( QgsRenderContext& rendererContext )
 
     select( attributes, rendererContext.extent() );
 
-    if ( mRendererV2->usingSymbolLevels() )
+    if ( ( mRendererV2->capabilities() & QgsFeatureRendererV2::SymbolLevels )
+         && mRendererV2->usingSymbolLevels() )
       drawRendererV2Levels( rendererContext, labeling );
     else
       drawRendererV2( rendererContext, labeling );

src/core/symbology-ng/qgscategorizedsymbolrendererv2.h

@@ -60,6 +60,10 @@ class CORE_EXPORT QgsCategorizedSymbolRendererV2 : public QgsFeatureRendererV2
 
     virtual QgsFeatureRendererV2* clone();
 
+    //! returns bitwise OR-ed capabilities of the renderer
+    //! \note added in 2.0
+    virtual int capabilities() { return SymbolLevels; }
+
     virtual QgsSymbolV2List symbols();
 
     const QgsCategoryList& categories() { return mCategories; }

src/core/symbology-ng/qgsgraduatedsymbolrendererv2.h

@@ -55,6 +55,10 @@ class CORE_EXPORT QgsGraduatedSymbolRendererV2 : public QgsFeatureRendererV2
 
     virtual QgsFeatureRendererV2* clone();
 
+    //! returns bitwise OR-ed capabilities of the renderer
+    //! \note added in 2.0
+    virtual int capabilities() { return SymbolLevels; }
+
     virtual QgsSymbolV2List symbols();
 
     QString classAttribute() const { return mAttrName; }

src/core/symbology-ng/qgsrendererv2.h

@@ -78,6 +78,14 @@ class CORE_EXPORT QgsFeatureRendererV2
     //! for debugging
     virtual QString dump();
 
+    enum Capabilities {
+      SymbolLevels = 1     // rendering with symbol levels (i.e. implements symbols(), symbolForFeature())
+    };
+
+    //! returns bitwise OR-ed capabilities of the renderer
+    //! \note added in 2.0
+    virtual int capabilities() { return 0; }
+
     //! for symbol levels
     virtual QgsSymbolV2List symbols() = 0;
 

src/core/symbology-ng/qgssinglesymbolrendererv2.h

@@ -36,6 +36,10 @@ class CORE_EXPORT QgsSingleSymbolRendererV2 : public QgsFeatureRendererV2
 
     virtual QgsFeatureRendererV2* clone();
 
+    //! returns bitwise OR-ed capabilities of the renderer
+    //! \note added in 2.0
+    virtual int capabilities() { return SymbolLevels; }
+
     virtual QgsSymbolV2List symbols();
 
     //! create renderer from XML element