clutter: Fix capture_into on non-origin view

The capture_into() function wrote out of bounds when capturing from a
non-origin view (not positioned at (0,0)). At the time of
implementation, this API was used to capture pixels across views into a
single data buffer, but the way it's used, and the way views may work
now, makes this impossible to do properly.

So remove the ability to capture into a pre-allocated buffer from
multiple views, and complain if the passed rectantgle overlapps with
multiple views. This removes the broken offset calculation all
together, fixing the bug motivating this change.

https://bugzilla.gnome.org/show_bug.cgi?id=787715