fixed build errors in GlVertexBuffer

Author: codex128

jme3-core/src/main/java/com/jme3/compat/package-info.java

@@ -0,0 +1,4 @@
+/**
+ * Contains utilities for Vulkan and OpenGL compatible applications.
+ */
+package com.jme3.compat;

jme3-core/src/main/java/com/jme3/scene/GlMesh.java

@@ -988,11 +988,16 @@ public void getTriangle(int index, Vector3f v1, Vector3f v2, Vector3f v3) {
      *
      * @param tri The triangle to store the positions in
      */
-    public void getTriangle(int index, Triangle tri) {
+    @Override
+    public Triangle getTriangle(int index, Triangle tri) {
+        if (tri == null) {
+            tri = new Triangle();
+        }
         getTriangle(index, tri.get1(), tri.get2(), tri.get3());
         tri.setIndex(index);
         tri.setCenter(null); // invalidate previously cached centroid, if any
         tri.setNormal(null);
+        return tri;
     }
 
     /**