SpeedTree 9 - Fix DoubleSided not enabled for non-billboard materials

- The HDRP property "Double-Sided" is not enabled for non-billboards materials.
- I've changed the `if` condition, since we cannot rely on the extra-map to determine if the material is a billboard or not.
- I also had to change the Double-Sided value, since it looks like `0.0f` = disable and `1.0f` = enabled. That's what I noticed when importing my ST9 assets in the editor...

Author: YohannVaastUnity

Packages/com.unity.render-pipelines.high-definition/Editor/Material/Nature/HDSpeedTree9MaterialUpgrader.cs

@@ -19,6 +19,7 @@ private static class HDProperties
             internal static readonly int CullModeForwardID = Shader.PropertyToID("_CullModeForward");
             internal static readonly int DiffusionProfileAssetID = Shader.PropertyToID("_Diffusion_Profile_Asset");
             internal static readonly int DiffusionProfileID = Shader.PropertyToID("_Diffusion_Profile");
+            internal static readonly int BillboardToggleID = Shader.PropertyToID("_BillboardKwToggle");
 
             internal static readonly string WindShared = "_WIND_SHARED";
             internal static readonly string WindBranch2 = "_WIND_BRANCH2";
@@ -53,10 +54,12 @@ private static void HDSpeedTree9MaterialFinalizer(Material mat)
 
         private static void SetupHDPropertiesOnImport(Material mat)
         {
-            // Since _DoubleSidedEnable controls _CullMode in HD,
-            // disable it for billboard LOD.
-            float doubleSided = mat.GetInt(HDProperties.ExtraMapKwToggleID) > 0 ? 0.0f : 1.0f; 
-            mat.SetFloat(HDProperties.DoubleSidedEnableID, doubleSided);
+            // Since _DoubleSidedEnable controls _CullMode in HD, disable it for billboard LOD.
+            if (mat.HasFloat(HDProperties.BillboardToggleID))
+            {
+                var isBillboard = mat.GetFloat(HDProperties.BillboardToggleID) == 1.0f;
+                mat.SetFloat(HDProperties.DoubleSidedEnableID, (isBillboard) ? 0.0f : 1.0f);
+            }
 
             if (mat.HasFloat(HDProperties.TwoSidedID))
             {