HLSL: add optional position.Y inversion

Adds command line options:

   --invert-y
   --iy

(synonyms) which invert position.Y on vertex shader output.  Handles these cases:

* Direct single variable return
* Member of direct returned struct
* Single variable output parameter
* Member of struct output parameter

API:

    // Enables position.Y output negation in vertex shader
    void TShader::setInvertY(bool invert);

Fixes #1173

Test/hlsl.y-negate-3.vert

@@ -0,0 +1,18 @@
+// Test Y negation from entry point out parameter
+
+float4 position;
+
+struct VS_OUT {
+    float4 pos : SV_Position;
+    int somethingelse;
+};
+
+VS_OUT main()
+{
+    VS_OUT vsout;
+
+    vsout.pos = position;
+    vsout.somethingelse = 42;
+
+    return vsout;
+}