buffer: hold tile cow mutex only when updating linked list
XXX: do not commit, buffer: do not use hot tile

probably makes some fast paths slower; determining better ways needs
profiling.
opt point-composer3 ops out of parallelization
operation: refactor in_place handling to take place in base classes

Adding a flag to operation, that is used by the implementation/further
subclass to declare that in-place processing should be done if possible.  This
also adds that capability to the base class of GIMPs layer modes.