fix the bug in MyVegetationGenerator class
change the class design of Renderable

Renderable is replaced by GeometryObject which represents each geometry object, such as a block, a parcel, and a building. Each geometry object has std::vector<Renderable*> as a class member. The new Renderable class represents a part of a geometry object which use one texture. If the geometry object uses only one texture, it has only one Renderable, while if it uses multiple textures, it has multiple Renderables. Also, some subclasses of Renderable are added, such as RenderableQuadList and RenderableTriangleList. These classes are helper classes to create vertex list.