Delay querying for EGL extension string until we are live with GL

Change-Id: I7317a5157d7692f6e3357479a5b4c0c7906c3f77
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Merge pull request #3 from mer-hybris/delay-egl-query

[qtscenegraph-adaptation] Delay querying for EGL extension string until we are live with GL. Fixes jb#22028