Fix inexplicably changed condition in GL backingstore composition

In a13e8d6660913bec172d1374f78083498c539df0 which provided the move
and decoupling of OpenGL code the !makeCurrent-succeeded &&
!context-is-valid condition got changed to !makeCurrent-succeeded &&
context-is-valid which makes no sense because what we test for is
context lost situations, in which case makeCurrent fails and isValid()
reports false.

Restore the ! that went missing in the above mentioned patch.

Change-Id: I94f84d2853bcbd2965f76ecd665d3614b1f0e746
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit c6c6e01e893b211f51511929b4258f789cdfd428)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/opengl/qplatformbackingstoreopenglsupport.cpp

@@ -174,7 +174,8 @@ void QPlatformBackingStoreOpenGLSupport::composeAndFlush(QWindow *window, const
 
     bool current = context->makeCurrent(window);
 
-    if (!current && context->isValid()) {
+    if (!current && !context->isValid()) {
+        // release resources and attempt to reinitialize upon context loss
         delete blitter;
         blitter = nullptr;
         textureId = 0;