Add dynamic generation of enviormental-map cube-map textures.

Add CC3GLEnvironmentMapTexture class as cube-map texture that can be dyanmically
and automatically rendered from snapshots of the scene. Cube-map textures now
loaded in expected orientation and flipped during loading to compensate for
internal GL storage of cube-maps. Add CC3GLTexture methods to flip the image
horizontally, or rotate 180 degrees. Adjust standard shaders to work with new
cube-map texture orientation. Fix issue with reading and writing color
content in cube-maps. Add reflective teapot demo to CC3DemoMashUp to show a
scene reflection automatically updating as objects move around the scene.
Color & depth buffer clearing now controlled by surfaces. Add PVR skybox option
to CC3DemoMashUp addSkybox method.