Summary:
The fact that the buffer is still referenced - that is used - when the
BufferInterface gets destroyed is an error, but it is not fatal.

Unfortunately KWin/Wayland is still hitting this assert from time to
time and the assert is not helping to find the cause as a backtrace
does not show where a reference is still hold.

This change removes the hard assert by a soft warning. The advantage
of the warning is that the compositor is not killed and that we can
observe the reason and find a usage pattern which triggers the condition.

With that we will hopefully be able to find the case where the buffer
is still referenced when being destroyed and fix that.

Reviewers: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D1783