Provide a system property to opt-out of using sun.misc.Unsafe

Property: joml.nounsafe

This also allows to use non-direct buffers, but is slower than using direct buffers with sun.misc.Unsafe.
This option is only provided for cases when JOML is used without an OpenGL binding library.

src/org/joml/MemUtil.java

@@ -35,12 +35,25 @@ of this software and associated documentation files (the "Software"), to deal
  * @author Kai Burjack
  */
 abstract class MemUtil {
+    private static final boolean nounsafe = hasOption("joml.nounsafe");
     static final MemUtil INSTANCE = createInstance();
 
+    private static boolean hasOption(String option) {
+    	String v = System.getProperty(option);
+    	if (v == null)
+    		return false;
+    	if (v.trim().length() == 0)
+    		return true;
+    	return Boolean.valueOf(v).booleanValue();
+    }
+
     private static final MemUtil createInstance() {
         MemUtil accessor;
         try {
-            accessor = new MemUtilUnsafe();
+            if (nounsafe)
+                accessor = new MemUtilNIO();
+            else
+                accessor = new MemUtilUnsafe();
         } catch (Throwable e) {
             accessor = new MemUtilNIO();
         }