Cubemap layout

Added the ability to pass a function to the ```TextureCubeMap::create()``` method to return the source area for each face within a single image. For example:

```auto cube = gl::TextureCubeMap::create ( loadImage ( "cubemap.jpg" ), gl::TextureCubeMap::Layout::VERTICAL );```

```TextureCubeMap::Layout::VERTICAL``` is a function that takes a cubemap face and returns a ```std::pair<ci::Area, ci::ivec2>```, where first is the srcArea in the image, and second is the relativeOffset. (used in ```Surface::copyFrom())