Make GLX and EGL dynamic dependencies for xcb

[ChangeLog][QPA][Xcb] GLX and EGL code paths are now dynamically
resolved, making it possible for one build of a plugin to use both code
paths. Default is to use the GLX code path if available. This can be
overwritten by specifying QT_XCB_GL_INTEGRATION=xcb_egl as an
evnironment variable. Enable qt.xcb.glintegration.debug to get debug log
output of what integration is used

Change-Id: Ia9fa95fcca3d901b91dadb8c98a695fea0ae3b1e
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
QOpenGLWindow: make it possible to use a shared context

Change-Id: I7e9f115a9b75d38c1d6a214958d18d6fd9eac891
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>