Fix the crash when using multihand tool with some complicated brushes

Right now the brush server returns the pointers to the brushes instead
of the copies of them, so when changing its parameters, the change is
shared among all their users. For now it is an acceptable behavior,
because even the Multihand Brush Tool uses the same brush for all the
"hands". But we should ensure that we don't reset internal state of
the brush when reloading it with the same config options.

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