Blender Git Loki

DrawManager: External Engine Depth Buffer

Only draw the depth buffer when overlays are enabled and scene
or view has changed.

When using Cycles in the viewport for every viewport draw call
the depth buffer was renewed. Draw calls happened when a sample
was finished or the status report was updated.

This could waist some CPU/GPU cycles. This change will check when
the depth buffer needs to be updated or when the last known depth
buffer could be reused.

Reviewers: brecht, fclem

Maniphest Tasks: T63525

Differential Revision: https://developer.blender.org/D4775