In gimp_gegl_apply_cached_operation(), when applying a non-point
filter with the same source and destination buffers, render the
result to a temporary buffer to avoid chunking artifacts.  We'd
previously duplicate the source buffer instead (with commit
35729ee0 erroneously copying the
cached results to the source/destination buffer before duplicating
it, causing this bug), but we now use a temporary result buffer
instead; this has roughly the same overhead, but would allow us to
keep the original operation-node input when committing a drawable
filter in a future commit, which would avoid dropping any cached
data.