no-arg constructor for Camera should be protected

[stephengold]

jmonkeyengine/jme3-core/src/main/java/com/jme3/renderer/Camera.java

Line 210 in b08649b

public Camera() {

The no-arg constructor doesn't fully initialize the camera, so it should be protected to discourage accidental use.

[stephengold]

As happened twice in jme3-vr!

[stephengold]

fixed in "master" branch at 0882742