Since commit 10b96b10, we had buffer leaks as soon as we ran filters.
gimp_drawable_get_buffer_with_effects() was either creating a new buffer (when a
drawable had filters) or not. And calling code couldn't know which is which.

Code needs to be consistent. Either we return a new reference to take ownership
to, or not. So let's change to add a new reference to the drawable' buffers so
that it always returns a new reference to free.