Blender Git Loki

Overlay: Wireframe: New method to avoid zfighting with geometry

This new method is only enabled if Overlay Smooth Wire is enabled.

This method gives really nice results but has some downside:
- Require a depth copy or loose the ability to write wire depth to the
depth buffer and have correct depth ordering of wires. This patch use the former, with its associated cost.
- Require some depth sampling and prevent early depth test (i.e: has
some performance impact).
- Has some relatively minor instability with geometry that are perpendicular
to the view and intersecting with other geometry.

Pros:
- Compared to a fullpass approach this is surely going to have less
performance impact and much higher quality.
- Removes the additional vertex offset. (see T74961)
- Fixes all half edges z-fighting.

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Reviewed By: brecht

Differential Revision: https://developer.blender.org/D7233