CoglTexture2DSliced: Pass slice tex to callback in foreach_sub_texture

The cogl_texture_foreach_sub_texture_in_region virtual for the sliced
texture backend was previously passing the CoglHandle of the sliced
texture to the callback. Since d5634e37 the slice texture backend now
works in terms of 2D textures so it's possible to pass the underlying
slice texture as a handle too. This makes all of the foreach callbacks
consistent in that they pass a CoglHandle of the primitive texture
type that matches the GL handle.

cogl/cogl-texture-2d-sliced.c

@@ -88,7 +88,7 @@ _cogl_texture_2d_sliced_foreach_cb (CoglHandle handle,
   virtual_coords_out[3] = (virtual_coords_in[3] * data->y_span->size +
                            data->y_span->start) / data->tex->height;
 
-  data->callback (data->tex,
+  data->callback (handle,
                   gl_handle,
                   gl_target,
                   slice_coords,