Fix test failing if compiler eliminates dead inputs

util/test/demos/vk/vk_spirv_13_shaders.cpp

@@ -35,7 +35,7 @@ void vertmain(in float3 pos : INPOS, in float4 col : INCOL, in float2 uv : UV,
               out float4 opos : SV_Position, out float4 outcol : COL)
 {
 	opos = float4(pos.xyz*float3(1,-1,1), 1);
-	outcol = col;
+	outcol = col + 1.0e-10 * uv.xyxy;
 }
 
 )EOSHADER";