[PATCH] QOpenGlContext: Always unset current context in doneCurrent()

[PATCH] QOpenGlContext: Always unset current context in doneCurrent()

Otherwise when no other context is made current until thread exit, the
QGuiGLThreadContext destructor will try to call doneCurrent() on an
already deleted context.

Change-Id: If55dd69a72b8ab4012780a449f6a02729dd0ed43
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit cd1686e55f706048286cbc962bbe02032c2396cd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 10c195b86432eaa430c6991c0fcb74c411407cdf)

Gbp-Pq: Name upstream_unset_current_openglcontext.diff