QSGRenderNode::matrix() returns a dangling pointer in QSGRenderNode::render()

Calling `QSGrenderNode::matrix` in `render()` returns a dangling pointer.

QSGRenderNode::m_matrix is assigned to a local stack variable in function `Renderer::prepareRhiRenderNode` (qsgbatchrender.cpp), but render() is called in `Renderer::renderRhiRenderNode`, so m_matrix now points to invalid stack space.

Besides, Imho there really is no valid reason why matrix() returns a pointer. Comes Qt7, that should just return a QMatrix4x4 so this doesn't happen again. Maybe adding a new method returning QMatrix4x4 should be considered.