Wayland-EGL QPA: Support desktop OpenGL as well

There is no hard requirement on OpenGL ES 2 in this plugin, so allow
Desktop OpenGL as well.

Change-Id: I5b58f21c3640a4403dc6aa710838d4904d85f482
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Only build Walyand EGL QPA if EGL is supported by base

Change-Id: I1e6e39e177d4c173b6eedf2c311bcfe0f9693705
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
QWayland-EGL: blacklist Mesa drivers from creating threaded GL

As many Mesa drivers have been shown to have trouble with threaded GL,
add this vendor to a threaded GL blacklist.

This can be overridden by setting QT_OPENGL_NO_SANITY_CHECK in the
environment.

Change-Id: I75b72a35cbbf92e958c1e7846488b1455ecc72d0
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>