This patch adds a new matte node that implements the Cryptomatte specification.
It also incluces a custom eye dropper that works outside of a color picker.
Cryptomatte export for the Cycles render engine will be in a separate patch.
Reviewers: brecht
Reviewed By: brecht
Subscribers: brecht
Tags: #compositing
Differential Revision: https://developer.blender.org/D3531
