[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/include/qmapboxgl.hpp

@@ -36,6 +36,11 @@ public:
         ConstrainWidthAndHeight
     };
 
+    enum ViewportMode {
+        DefaultViewport = 0,
+        FlippedYViewport
+    };
+
     MapMode mapMode() const;
     void setMapMode(MapMode);
 
@@ -45,6 +50,9 @@ public:
     ConstrainMode constrainMode() const;
     void setConstrainMode(ConstrainMode);
 
+    ViewportMode viewportMode() const;
+    void setViewportMode(ViewportMode);
+
     unsigned cacheDatabaseMaximumSize() const;
     void setCacheDatabaseMaximumSize(unsigned);
 
@@ -61,6 +69,7 @@ private:
     MapMode m_mapMode;
     GLContextMode m_contextMode;
     ConstrainMode m_constrainMode;
+    ViewportMode m_viewportMode;
 
     unsigned m_cacheMaximumSize;
     QString m_cacheDatabasePath;

platform/qt/src/qmapboxgl.cpp

@@ -35,6 +35,10 @@ static_assert(mbgl::underlying_type(QMapboxGLSettings::NoConstrain) == mbgl::und
 static_assert(mbgl::underlying_type(QMapboxGLSettings::ConstrainHeightOnly) == mbgl::underlying_type(mbgl::ConstrainMode::HeightOnly), "error");
 static_assert(mbgl::underlying_type(QMapboxGLSettings::ConstrainWidthAndHeight) == mbgl::underlying_type(mbgl::ConstrainMode::WidthAndHeight), "error");
 
+// mbgl::ViewportMode
+static_assert(mbgl::underlying_type(QMapboxGLSettings::DefaultViewport) == mbgl::underlying_type(mbgl::ViewportMode::Default), "error");
+static_assert(mbgl::underlying_type(QMapboxGLSettings::FlippedYViewport) == mbgl::underlying_type(mbgl::ViewportMode::FlippedY), "error");
+
 // mbgl::NorthOrientation
 static_assert(mbgl::underlying_type(QMapboxGL::NorthUpwards) == mbgl::underlying_type(mbgl::NorthOrientation::Upwards), "error");
 static_assert(mbgl::underlying_type(QMapboxGL::NorthRightwards) == mbgl::underlying_type(mbgl::NorthOrientation::Rightwards), "error");
@@ -61,6 +65,7 @@ QMapboxGLSettings::QMapboxGLSettings()
     : m_mapMode(QMapboxGLSettings::ContinuousMap)
     , m_contextMode(QMapboxGLSettings::SharedGLContext)
     , m_constrainMode(QMapboxGLSettings::ConstrainHeightOnly)
+    , m_viewportMode(QMapboxGLSettings::DefaultViewport)
     , m_cacheMaximumSize(mbgl::util::DEFAULT_MAX_CACHE_SIZE)
     , m_cacheDatabasePath(":memory:")
     , m_assetPath(QCoreApplication::applicationDirPath())
@@ -97,6 +102,16 @@ void QMapboxGLSettings::setConstrainMode(ConstrainMode mode)
     m_constrainMode = mode;
 }
 
+QMapboxGLSettings::ViewportMode QMapboxGLSettings::viewportMode() const
+{
+    return m_viewportMode;
+}
+
+void QMapboxGLSettings::setViewportMode(ViewportMode mode)
+{
+    m_viewportMode = mode;
+}
+
 unsigned QMapboxGLSettings::cacheDatabaseMaximumSize() const
 {
     return m_cacheMaximumSize;
@@ -605,7 +620,8 @@ QMapboxGLPrivate::QMapboxGLPrivate(QMapboxGL *q, const QMapboxGLSettings &settin
         *this, *fileSourceObj,
         static_cast<mbgl::MapMode>(settings.mapMode()),
         static_cast<mbgl::GLContextMode>(settings.contextMode()),
-        static_cast<mbgl::ConstrainMode>(settings.constrainMode())))
+        static_cast<mbgl::ConstrainMode>(settings.constrainMode()),
+        static_cast<mbgl::ViewportMode>(settings.viewportMode())))
 {
     qRegisterMetaType<QMapboxGL::MapChange>("QMapboxGL::MapChange");
 

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();

platform/qt/src/qquickmapboxglrenderer.cpp

@@ -16,6 +16,7 @@ QQuickMapboxGLRenderer::QQuickMapboxGLRenderer()
     settings.setAccessToken(qgetenv("MAPBOX_ACCESS_TOKEN"));
     settings.setCacheDatabasePath("/tmp/mbgl-cache.db");
     settings.setCacheDatabaseMaximumSize(20 * 1024 * 1024);
+    settings.setViewportMode(QMapboxGLSettings::FlippedYViewport);
 
     m_map.reset(new QMapboxGL(nullptr, settings));
 }