Add support for querying "display" from eglfs

Return the native display which, with the x11 hooks, is the X11 Display*.
This gives compatibility with xcb and allows QtWebEngine to run on eglfs
in regular desktop builds without ozone. (this requires eglfs built with the
x11 hooks of course)

Change-Id: I8d25e2835c41a7e78f063a57a7d6c6b5e53a19b4
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Support adopting an existing EGLContext in eglfs and xcb

Add also a manual test application. For GLX there is an autotest since
that is likely to be run on one of the CIs. For EGL and especially
eglfs this is likely not the case so a manual test is better.

Task-number: QTBUG-37552
Change-Id: Ib09db5d909befb68d16f69abd401a56abe55f28a
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>