GL_EXT_mesh_shader: clarify if interpolation qualifier can be used for per-primitive input/output

[yuq]

I can't find explicit words in spec for this topic. Interpolation qualifier (flat, smooth...) is not allowed to be used with "patch" qualifier in 4.6 spec:

The auxiliary storage qualifier patch is not used for interpolation; it is a compile-time error to use
interpolation qualifiers with patch.

So I expect interpolation qualifier should not be used for per-primitive IO as it makes no sense to interpolate per-primitive IO. But glslang compiler allow this usage (no compiler error).