HLSL: Flatten vertex input and fragment output structures.

Vulkan can't handle structures into the vertex stage or out
of the fragment stage.

Test/hlsl.structin.vert

@@ -0,0 +1,9 @@
+struct VI {
+    float4 m[2];
+    uint2 coord;
+};
+
+float4 main(float4 d, VI vi, float4 e) : SV_POSITION
+{
+    return vi.m[1] + vi.m[0] + float4(vi.coord.x) + d + e;
+}