Blender Git Loki

Fix T81169: Grease Pencil Z-depth drawing issue on OSX + AMD Graphic Cards

The grease pencil merge depth shader is designed to only work correctly
in octographic mode. The uv coordinates used `noperspective` attribute.
Somehow this doesn't lead to render artifacts on most platforms and was
only detected on OSX + AMD cards.

This fix would calculate the uv coordinate inside the fragment shader
and isn't passed along from the vertex shader.

Thanks to Sebasti�n Barschkis for providing the hardware and time and
Cl�ment Foucault for helping out with the final fix.