Fix non-standard device orientation values

Yaw and roll were mixed up. Previously 'roll' was assigned to 'alpha' and 'yaw' was assigned to 'gamma'. Fixed to assign 'yaw' to 'alpha' and 'roll' to 'gamma', matching the standard behavior with Safari iOS 7.1 on
iPhone and Chrome for Android 4.4.

This is a breaking change for existing Ejecta projects, but I believe Ejecta ought to be made standards-compliant, especially since it is trivial to fix existing projects: just swap the 'alpha' and 'gamma' values, and then the game will also be compatible with standards-compliant mobile browsers.