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-2.vert

@@ -0,0 +1,8 @@
+// Test Y negation from entry point out parameter
+
+float4 pos;
+
+void main(out float4 position : SV_Position)
+{
+    position = pos;
+}