Silence maybe_uninitialized warning with gcc -Og

GCC produces false positives for maybe_uninitialized when compiling with
-Og in these three places. Simply initialize the variables to silence
it. This should be entirely cost-free for normal compilation.

Change-Id: Iab778a6ba25993f78f190e928c1fcc2dbd8b2fcd
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/painting/qtriangulatingstroker.cpp

@@ -321,7 +321,7 @@ void QTriangulatingStroker::cubicTo(const qreal *pts)
     if (threshold < 4)
         threshold = 4;
     qreal threshold_minus_1 = threshold - 1;
-    float vx, vy;
+    float vx = 0, vy = 0;
 
     float cx = m_cx, cy = m_cy;
     float x, y;

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -417,7 +417,7 @@ void QXcbConnection::initializeScreens()
                 qWarning("failed to get the current screen resources");
                 free(error);
             } else {
-                xcb_timestamp_t timestamp;
+                xcb_timestamp_t timestamp = 0;
                 xcb_randr_output_t *outputs = Q_NULLPTR;
                 int outputCount = xcb_randr_get_screen_resources_current_outputs_length(resources_current.data());
                 if (outputCount) {

src/widgets/kernel/qapplication.cpp

@@ -3305,7 +3305,7 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
                 QWheelEvent we(relpos, wheel->globalPos(), wheel->pixelDelta(), wheel->angleDelta(), wheel->delta(), wheel->orientation(), wheel->buttons(),
                                wheel->modifiers(), phase, wheel->source(), wheel->inverted());
                 bool eventAccepted;
-                while (w) {
+                do {
                     we.spont = spontaneous && w == receiver;
                     we.ignore();
                     res = d->notify_helper(w, &we);
@@ -3323,7 +3323,7 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
 
                     we.p += w->pos();
                     w = w->parentWidget();
-                }
+                } while (w);
                 wheel->setAccepted(eventAccepted);
             } else if (!spontaneous) {
                 // wheel_widget may forward the wheel event to a delegate widget,