layer: fix ownership when failing to add clip

layer: fix ownership when failing to add clip

If a clip is already part of a layer, then adding it to another layer
should fail. Previously, in this case, `ges_layer_add_clip` was adding a
reference to the clip instead, without subsequently giving up ownership.
This meant that the clip would be left with an unowned reference.
This has now been corrected by also calling `unref` after the
`ref_sink`.

Note that, since `clip` is already part of `current_layer`, it should
already be non-floating, so the `ref_sink`-`unref` should do nothing
overall. But we keep both to make the ownership (transfer floating/none)
explicit.