Add default initialisation to various multimedia classes

Change-Id: I902de05984fdae152e2678e4cf2d401a0b670703
Reviewed-by: Jonas Rabbe <jonas.rabbe@gmail.com>

src/imports/multimedia/qsgvideonode_rgb.cpp

@@ -76,6 +76,9 @@ class QSGVideoMaterialShader_RGB : public QSGMaterialShader
 public:
     QSGVideoMaterialShader_RGB(QVideoFrame::PixelFormat pixelFormat)
         : QSGMaterialShader(),
+          m_id_matrix(-1),
+          m_id_rgbTexture(-1),
+          m_id_opacity(-1),
           m_pixelFormat(pixelFormat)
     {
     }