Add recentering support for Oculus Rift, improve associated Javadoc

jme3-vr/src/main/java/com/jme3/input/vr/OculusVR.java

@@ -329,12 +329,12 @@ public class OculusVR implements VRAPI {
 
     @Override
     public void reset() {
-        throw new UnsupportedOperationException();
+        ovr_RecenterTrackingOrigin(session);
     }
 
     @Override
     public void getRenderSize(Vector2f store) {
-        if(!isInitialized()) {
+        if (!isInitialized()) {
             throw new IllegalStateException("Cannot call getRenderSize() before initialized!");
         }
         store.x = textureW;

jme3-vr/src/main/java/com/jme3/input/vr/VRAPI.java

@@ -86,7 +86,8 @@ public interface VRAPI {
     public boolean isInitialized();
 
     /**
-     * Reset the VR system.
+     * Reset (recenter) the VR system. The current position of the HMD is
+     * now considered the origin (observer+[0,0,0]).
      */
     public void reset();