shaped-texture: never slice shape mask texture

Since Cogl doesn't support multi-texturing with sliced textures and the
shape texture is combined with the texture-from-pixmap texture we need
to make sure we never construct a sliced mask texture. This patch simply
passes the COGL_TEXTURE_NO_SLICE flag to cogl_texture_from_data when
creating the shape mask texture.

https://bugzilla.gnome.org/show_bug.cgi?id=674731