Fix atan2 argument order for HLSL (#1712)

Signed-off-by: Doug Walker <doug.walker@autodesk.com>

Signed-off-by: Doug Walker <doug.walker@autodesk.com>
(cherry picked from commit 8a93946)
Signed-off-by: Cédrik Fuoco <cedrik.fuoco@autodesk.com>

src/OpenColorIO/GpuShaderUtils.cpp

@@ -1072,8 +1072,10 @@ std::string GpuShaderText::atan2(const std::string & y,
         }
         case GPU_LANGUAGE_HLSL_DX11:
         {
-            // note: operand order is swapped in HLSL
-            kw << "atan2(" << x << ", " << y << ")";
+            // note: Various internet sources claim that the x & y arguments need to be
+            // swapped for HLSL (relative to GLSL).  However, recent testing on Windows
+            // has revealed that the argument order needs to be the same as GLSL.
+            kw << "atan2(" << y << ", " << x << ")";
             break;
         }
         case LANGUAGE_OSL_1: