Fix memory leaks&uninitialized members in QQuickContext2D&QQuickContext2DTexture

1) Delete m_buffer in QQuickContext2D::~QQuickContext2D
2) Add a default ctor for QQuickContext2D::State, although the reset() method is
   always called, but the valgrind still complains the "uninitialized" error, so
   add this to make an explicit initialization.
3) m_item in QQuickContext2DTexture is also not initialized.

Change-Id: Ie6c74136342d7f0ff8dc268d5d6b976c95ff52f1
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>

src/declarative/items/context2d/qquickcontext2d.cpp

@@ -3307,6 +3307,7 @@ QQuickContext2D::QQuickContext2D(QQuickCanvasItem* item)
 
 QQuickContext2D::~QQuickContext2D()
 {
+    delete m_buffer;
 }
 
 v8::Handle<v8::Object> QQuickContext2D::v8value() const

src/declarative/items/context2d/qquickcontext2d_p.h

@@ -107,6 +107,30 @@ public:
 
 
     struct State {
+        State()
+            : strokeStyle(QColor("#000000"))
+            , fillStyle(QColor("#000000"))
+            , fillPatternRepeatX(false)
+            , fillPatternRepeatY(false)
+            , strokePatternRepeatX(false)
+            , strokePatternRepeatY(false)
+            , fillRule(Qt::WindingFill)
+            , globalAlpha(1.0)
+            , lineWidth(1)
+            , lineCap(Qt::FlatCap)
+            , lineJoin(Qt::MiterJoin)
+            , miterLimit(10)
+            , shadowOffsetX(0)
+            , shadowOffsetY(0)
+            , shadowBlur(0)
+            , shadowColor(qRgba(0, 0, 0, 0))
+            , globalCompositeOperation(QPainter::CompositionMode_SourceOver)
+            , font(QFont(QLatin1String("sans-serif"), 10))
+            , textAlign(QQuickContext2D::Start)
+            , textBaseline(QQuickContext2D::Alphabetic)
+        {
+        }
+
         QTransform matrix;
         QPainterPath clipPath;
         QBrush strokeStyle;

src/declarative/items/context2d/qquickcontext2dtexture.cpp

@@ -72,6 +72,7 @@ Q_GLOBAL_STATIC(QThread, globalCanvasThreadRenderInstance)
 QQuickContext2DTexture::QQuickContext2DTexture()
     : QSGDynamicTexture()
     , m_context(0)
+    , m_item(0)
     , m_canvasSize(QSize(1, 1))
     , m_tileSize(QSize(1, 1))
     , m_canvasWindow(QRect(0, 0, 1, 1))