Do not overwrite existing event mask of root window

QXcbScreen installs its own event mask on the screen's root window.
This overwrites any existing event mask already set and by that
breaks applications when a new screen is added.

By first fetching the existing event mask and adding it to the newly
installed event mask, Qt does no longer break applications also
installing an event mask on the root window.

Task-number: QTBUG-39648
Change-Id: I8686dd6ae49d0e807c6fe1ea4a231ff728bfcf25
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Properly check which OpenGL features are supported

QOpenGLShaderProgram::hasOpenGLShaderPrograms tests whether
QOpenGLFunctions::Shaders is provided for the given context. As the
initialization code assumed OpenGL 2 this always was true. But
unfortunately we still cannot assume that OpenGL 2 is universally
supported. E.g. indirect rendering (no matter how bad that idea is)
does not support OpenGL 2 on all hardware and the Shader related
extensions are not available.

This change makes sure that only when OpenGL 2 is available the
features provided by OpenGL 2 are enabled. If OpenGL 2 is not
available the extensions are tested. The checks are slightly
reordered to not do the extension tests at all if OpenGL 2 is
available.

Task-number: QTBUG-39730
Change-Id: Ic775163e0dcc519925b1287f3c4ca5e8ebf101d5
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
pass a pointer instead of a reference to initFrom()

msvc thinks that it's impossible to create a null reference (because
some language lawyer said so) and thus complains about our assert that
checks the reference's validity. work around by not dereferencing the
pointers we already have.

Change-Id: Ife2288d4187860105de12fdebc0e671e0159ace3
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>