[Qt] Added viewport mode

Qt Quick requires third-party rendering codes to follow its standards
with regards to viewport transform. Qt 5.6 adds a 'mirrorVertically'
property but that only solves the solution for that version of Qt
onwards. Telling Mapbox GL core to flip the Y axis makes QQuickMapboxGL
item available for Qt 5.2 onwards.

Qt 'mirrorVertically' property documentation:
http://doc.qt.io/qt-5/qquickframebufferobject.html#mirrorVertically-prop

Fixes #4866.

platform/qt/src/qquickmapboxgl.cpp

@@ -14,10 +14,6 @@
 QQuickMapboxGL::QQuickMapboxGL(QQuickItem *parent_)
     : QQuickFramebufferObject(parent_)
 {
-#if QT_VERSION >= 0x050600
-    // FIXME: https://github.com/mapbox/mapbox-gl-native/issues/4866
-    setMirrorVertically(true);
-#endif
 }
 
 QQuickMapboxGL::~QQuickMapboxGL()
@@ -166,7 +162,7 @@ QColor QQuickMapboxGL::color() const
 
 void QQuickMapboxGL::pan(int dx, int dy)
 {
-    m_pan += QPointF(dx, dy);
+    m_pan += QPointF(dx, -dy);
 
     m_syncState |= PanNeedsSync;
     update();