[sailfishos][embedlite][egl] Fix mesa egl display initialisation
On mesa based systems, the egl display does not like being initialised twice.
This patch retrieves the egl display from qtmozembed and uses that. A related change is required in qtmozembed to return the egl display.
This fixes the browser, but not the email app, because the browser uses an external context. The rendering path for the email app is different inside gecko.