Blender Git Loki

Fix T74694: Sculpt Mask Influences Weight/Vertex Paint Drawing

Sculpt overlay assumed that the sculpt session was always in sculpt
mode. But the sculpt session was also used for Vertex/Weight painting.
This resulted that when a mask was defined in the sculpt mode this
resulted into render artifacts.

This patch adds a check to see if the object has a sculpt session for
OB_MODE_SCULPT.

Reviewed By: Cl�ment Foucault

Differential Revision: https://developer.blender.org/D7704