This patch adds the core realtime compositor evaluator as well as a compositor draw engine powered by the evaluator that operates in the viewport. The realtime compositor is a new GPU accelerated compositor that will be used to power the viewport compositor imminently as well as the existing compositor in the future. This patch only adds the evaluator and engine as an experimental feature, the implementation of the nodes themselves will be committed separately. See T99210. Differential Revision: https://developer.blender.org/D15206 Reviewed By: Clement Foucault