Introduce new platform capability ThreadedOpenGL.

Introduce new platform capability ThreadedOpenGL.

Lets the platform plugin advertise whether it's safe to use OpenGL from
a different thread. With XCB we only advertise this if we have a
reasonably new XCB libary, as older versions suffer from the
xcb_wait_for_reply() blocking bug, which cause GL rendering in a
separate to stall when using Mesa drivers.

Change-Id: I4829df7e583a1c8aed218ae13a159d21266cc594
Reviewed-on: http://codereview.qt.nokia.com/3613
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>