Fix memory leaks&uninitialized members in QQuickContext2D&QQuickContext2DTexture

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>