Add shader keyword support

Source/ConformalDecals/MaterialProperties/MaterialKeywordProperty.cs

@@ -0,0 +1,19 @@
+using ConformalDecals.Util;
+using UnityEngine;
+
+namespace ConformalDecals.MaterialProperties {
+    public class MaterialKeywordProperty : MaterialProperty {
+        [SerializeField] public bool value = true;
+        
+        public override void ParseNode(ConfigNode node) {
+            base.ParseNode(node);
+
+            ParseUtil.ParseBoolIndirect(ref value, node, "value");
+        }
+
+        public override void Modify(Material material) {
+            if (value) material.EnableKeyword(_propertyName);
+            else material.DisableKeyword(_propertyName);
+        }
+    }
+}

Source/ConformalDecals/MaterialProperties/MaterialTextureProperty.cs

@@ -67,6 +67,7 @@ namespace ConformalDecals.MaterialProperties {
             material.SetTexture(_propertyID, _texture);
             material.SetTextureOffset(_propertyID, _textureOffset);
             material.SetTextureScale(_propertyID, _textureScale * _scale);
+            if (_propertyName != "_Decal") material.EnableKeyword("DECAL" + _propertyName.ToUpper());
         }
 
         public void SetScale(Vector2 scale) {