Blender Git Loki

Modifiers: Refactor Mask modifier

The functionality of the mask modifier remains unchanged.

This patch updates the mask modifier so that it uses C++.
The manual memory management has been replaced with proper containers.
The large `applyModifier` function has been splitup into multiple smaller functions.
A large speedup is achieved by using simple arrays instead of hash tables in multiple places.

In my performance test file the playback speed increased from 1.1 to 5.1 fps on my laptop.

Reviewers: campbellbarton, brecht

Differential Revision: https://developer.blender.org/D6779