Accelerate path operations by disabling canvas redrawing during the operation

Accelerate the break-apart and boolean path operations on larger paths by disabling redrawing the canvas during running of the algorithms.

Testing this change with a large pathset accelerated the break-apart operation duration from 24 seconds down to mere 4 seconds, and cut-path operation duration to a fraction of original.

Signed-off-by: Olli Parviainen oparviai@iki.fi