gl/ios: Create a GL subview internally
Submitted by Sebastian Dröge
Currently we require the application to provide us with a UIView that has a CEAGLLayer as its layer. This is not very obvious to users and we really don't have to require that.
The alternative would be to do the same we do on OSX already: create a UIView with a CEAGLLayer as layer internally, and then add it as a subview to the UIView that is passed by the application. This change should also be backwards compatible.