It doesn't make sense to allow the window texture to be sliced,
but not allow the mask texture to be sliced. So, we should either
set 'disable-slicing' for the texture actor or not pass NO_SLICING when
creating the mask texture.

While slicing currently results in only the first layer being shown,
disabling slicing entirely would break windows larger than the texture
size limit.  It would be straightforward to support multitexturing of
sliced textures in Cogl when all the textures are identically sized
and sliced identically, if it became a priority.