OpenGL & Vulkan has workarounds when gl_Layer/gl_ViewportIndex isn't
supported. In this case a geometry shader will is generated. This
geometry shader doesn't follow the GLSL standard and doesn't work on
some platforms. This has not been an issue as the platforms that
don't support gl_Layer/gl_ViewportIndex don't show the issue.
According to the specs gl_Layer and gl_ViewportIndex should be set for
each call to EmitVertex. A shader should not rely on that EmitVertex
reuses the same memory.
Ref https://www.khronos.org/opengl/wiki/Geometry_Shader#Layered_rendering
```
Warning: gl_Layer and gl_ViewportIndex are GS output variables. As such, every time
you call EmitVertex, their values will become undefined. Therefore, you must set
these variables every time you loop over outputs.
```
Issue detected during development of !129062
Pull Request: https://projects.blender.org/blender/blender/pulls/130506
4018e52d30