vkd3d-shader: Create a separate variable for output varyings.

Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Matteo Bruni <mbruni@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

tests/hlsl-invalid.shader_test

@@ -146,3 +146,16 @@ float4 main(struct input i) : sv_target
 {
     return i.a;
 }
+
+[pixel shader fail]
+struct output
+{
+    float4 t : sv_target;
+    int a;
+};
+
+void main(out struct output o)
+{
+    o.t = float4(0, 0, 0, 0);
+    o.a = 0;
+}