Adjust demos and add cube mapping demo

demos/CMakeLists.txt

demos/CubeMapping/.gitignore

demos/CubeMapping/CMakeLists.txt

demos/CubeMapping/shaders/CubeMapReflection.frag

demos/CubeMapping/shaders/CubeMapReflection.vert

demos/CubeMapping/shaders/SkyBox.frag

demos/CubeMapping/shaders/SkyBox.vert

demos/CubeMapping/src/main.cpp

demos/InstancingDemo/src/main.cpp

demos/NormalMapping/src/main.cpp

framework