Highly optimized Displacement option for the Experiment PaintOp

This patch re-enables the Displacement option for the Experiment
PaintOp as was requested by Lukas. The new implementation has two
kinds of optimizations:
1) It updates big shapes (128+ pixes) using diffs of paths. It means
that the whole shape is not rendered every time. Only the changed
part of it is rendered. This gives up to 2 times better performance
for huge shapes of 2000+ px in size.
2) The path is incrementally simplified. That is the elements smaller
than 1% of the shape are truncated to a single line.