Ell authored Dec 13, 2017 and Øyvind "pippin" Kolås committed Dec 14, 2017

Use gegl_operation_context_dup_input_maybe_copy(), added in the
previous commit, in GimpOperationFilter, GimpOperationComposer,
GimpOperationComposer3, and OpTransform (however, *not* in the
corresponding point ops), to (potentially) create a separate copy
of the input buffer for each thread, to avoid lock contention over
the input buffer's tile-storage lock.  Use the input buffer
directly only for the first chunk, which is processed by the caller
thread.

This significantly improves performance of operations that randomly
access the input buffer (e.g., using a sampler), and seems not to
pessimize operations with a more regular access pattern (e.g.,
using an iterator, or scan rows).