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-1.vert

@@ -0,0 +1,9 @@
+
+// Test Y negation from entry point return
+
+float4 pos;
+
+float4 main() : SV_Position
+{
+    return pos;
+}