Initialize textureId in platform backing store

Setting it initially to 0 is very important, otherwise we will
do a glDeleteTextures with the undefined value.

The result sometimes goes unnoticed and sometimes causes bizarre
issues: For example in the 'textures' example one face of one cube
out of the six did go blank from time to time since the corresponding
texture was deleted by the backingstore.

Change-Id: Id19eb2164471b542b08a277a65edfcb5d0f8248d
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/gui/painting/qplatformbackingstore.cpp

@@ -62,6 +62,7 @@ public:
     QPlatformBackingStorePrivate(QWindow *w)
         : window(w)
 #ifndef QT_NO_OPENGL
+        , textureId(0)
         , blitter(0)
 #endif
     {