SDLPP: add constant rotation

sdlpp/sdlpp_renderobject.hpp

@@ -131,10 +131,16 @@ public:
     void setRotationCenter( const Vec2D< double > &center );
     void rotateClockwise( int angle );
     void rotateCounterClockwise( int angle );
+    void setRotationSpeed(int speed);
+    void startRotation();
+    void stopRotation();
 
 protected:
     virtual void copyTo( std::shared_ptr< RenderObject > other );
     bool entireTexture();
+    bool isRotating() {
+        return rotating;
+    }
     std::vector< std::shared_ptr< CollisionPolygon > > collisions;
     std::shared_ptr< Texture > texture;
     std::shared_ptr< Texture > cur_texture;
@@ -164,6 +170,8 @@ protected:
     Vec2D< double > rotation_center = { -1, -1 };
     SDL_Point rotation_center_point;
     int rotation_angle = 0;
+    int rotation_speed = 720;
+    bool rotating = false;
 
 private:
     friend Scene;