Blender Git Loki

Fix T73518: Normal Overlay

This change will not render the normals for faces that are hidden.
Before we had instance drawing the hidden faces were registered in the
index buffer. During the overlay refactoring the rendering was migrated
to instance rendering. Instance rendering does not use the index buffer
so the data was ignored.

This patch stored the normal visibility in the .w part of the normal or
for face normals it will set the normal to zero. The shader looks at
this and renders the normals fully transparent when detected.

Reviewed By: Cl�ment Foucault

Differential Revision: https://developer.blender.org/D6798