Fix flat interpolation check on fs inputs qualified with perPrimitiveNV

[dreamelon]

Spec allows any interpolation qualifier on fragment shader inputs qualified with "perprimitiveNV".
https://github.com/KhronosGroup/GLSL/blob/d368fe808d7e11476945901120510edbe490f961/extensions/nv/GLSL_NV_mesh_shader.txt#L1119

[dreamelon]

Ping for review

[dnovillo]

The fix itself LGTM, but this needs a test. Please add a test that exercises this path. Maybe something like

#version 450
#extension GL_NV_mesh_shader : require
layout(location = 0) perprimitiveNV in int idx;
layout(location = 0) out vec4 fragColor;
void main() { fragColor = vec4(float(idx)); }

[dnovillo]

@dreamelon are you still working on this PR?

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