Fix RenderState hash invalidation in stencil setters and flipFaceCull (#2603)

* Initial plan

* Fix RenderState hash invalidation in stencil setters and flipFaceCull

Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

Author: Copilot

jme3-core/src/main/java/com/jme3/material/RenderState.java

@@ -1165,6 +1165,7 @@ public TestFunction getBackStencilFunction() {
      */
     public void setFrontStencilMask(int frontStencilMask) {
         this.frontStencilMask = frontStencilMask;
+        cachedHashCode = -1;
     }
 
     /**
@@ -1174,6 +1175,7 @@ public void setFrontStencilMask(int frontStencilMask) {
      */
     public void setBackStencilMask(int backStencilMask) {
         this.backStencilMask = backStencilMask;
+        cachedHashCode = -1;
     }
 
     /**
@@ -1183,6 +1185,7 @@ public void setBackStencilMask(int backStencilMask) {
      */
     public void setFrontStencilReference(int frontStencilReference) {
         this.frontStencilReference = frontStencilReference;
+        cachedHashCode = -1;
     }
 
     /**
@@ -1192,6 +1195,7 @@ public void setFrontStencilReference(int frontStencilReference) {
      */
     public void setBackStencilReference(int backStencilReference) {
         this.backStencilReference = backStencilReference;
+        cachedHashCode = -1;
     }
 
     /**
@@ -1789,8 +1793,8 @@ public String toString() {
      */
     public void flipFaceCull() {
         switch (cullMode) {
-            case Back:  cullMode = FaceCullMode.Front; break;
-            case Front: cullMode = FaceCullMode.Back;  break;
+            case Back:  setFaceCullMode(FaceCullMode.Front); break;
+            case Front: setFaceCullMode(FaceCullMode.Back);  break;
         }
     }
 

jme3-core/src/test/java/com/jme3/material/RenderStateTest.java

@@ -53,6 +53,57 @@ public class RenderStateTest {
     // *************************************************************************
     // new methods exposed
 
+    @Test
+    public void testHashInvalidation() {
+        /*
+         * Test that setters properly invalidate the cached hash code.
+         */
+        RenderState state = new RenderState();
+        
+        // Get initial hash to force caching
+        int hash1 = state.contentHashCode();
+        
+        // Test setFrontStencilMask invalidates hash
+        state.setFrontStencilMask(0x12345678);
+        int hash2 = state.contentHashCode();
+        Assert.assertNotEquals("setFrontStencilMask should invalidate hash", hash1, hash2);
+        
+        // Test setBackStencilMask invalidates hash
+        hash1 = state.contentHashCode();
+        state.setBackStencilMask(0x87654321);
+        hash2 = state.contentHashCode();
+        Assert.assertNotEquals("setBackStencilMask should invalidate hash", hash1, hash2);
+        
+        // Test setFrontStencilReference invalidates hash
+        hash1 = state.contentHashCode();
+        state.setFrontStencilReference(42);
+        hash2 = state.contentHashCode();
+        Assert.assertNotEquals("setFrontStencilReference should invalidate hash", hash1, hash2);
+        
+        // Test setBackStencilReference invalidates hash
+        hash1 = state.contentHashCode();
+        state.setBackStencilReference(99);
+        hash2 = state.contentHashCode();
+        Assert.assertNotEquals("setBackStencilReference should invalidate hash", hash1, hash2);
+        
+        // Test flipFaceCull invalidates hash
+        state.setFaceCullMode(RenderState.FaceCullMode.Back);
+        hash1 = state.contentHashCode();
+        state.flipFaceCull();
+        hash2 = state.contentHashCode();
+        Assert.assertNotEquals("flipFaceCull should invalidate hash", hash1, hash2);
+        Assert.assertEquals("flipFaceCull should flip Back to Front", 
+                RenderState.FaceCullMode.Front, state.getFaceCullMode());
+        
+        // Test flipFaceCull again (Front to Back)
+        hash1 = state.contentHashCode();
+        state.flipFaceCull();
+        hash2 = state.contentHashCode();
+        Assert.assertNotEquals("flipFaceCull should invalidate hash (Front to Back)", hash1, hash2);
+        Assert.assertEquals("flipFaceCull should flip Front to Back", 
+                RenderState.FaceCullMode.Back, state.getFaceCullMode());
+    }
+
     @Test
     public void testCloneRenderState() {
         for (RenderState.BlendEquation equation : RenderState.BlendEquation.values()) {