st/private: Fix texture leak when creating shadow from actor (LP: 2121786)

cogl_pipeline_set_layer_texture() adds its own ref to the texture in _cogl_pipeline_set_layer_texture_data() when called via _st_create_shadow_pipeline(), so the initial reference is leaked when the pointer is stolen.

Fix this by not stealing the pointer, so autoptr can unref the texture. Before the code was changed to autoptr in 71accdd9 the texture was also manually unrefed after calling _st_create_shadow_pipeline().