RenderManager: cleanup viewport computation (related to PR #217)

jme3-core/src/main/java/com/jme3/renderer/RenderManager.java

@@ -865,8 +865,10 @@ public class RenderManager {
         if (cam != prevCam || cam.isViewportChanged()) {
             viewX      = (int) (cam.getViewPortLeft() * cam.getWidth());
             viewY      = (int) (cam.getViewPortBottom() * cam.getHeight());
-            viewWidth = ((int)(cam.getViewPortRight() * cam.getWidth())) - ((int)(cam.getViewPortLeft() * cam.getWidth()));
-            viewHeight = ((int)(cam.getViewPortTop() * cam.getHeight())) - ((int)(cam.getViewPortBottom() * cam.getHeight()));
+            int viewX2 = (int) (cam.getViewPortRight() * cam.getWidth());
+            int viewY2 = (int) (cam.getViewPortTop() * cam.getHeight());
+            viewWidth  = viewX2 - viewX;
+            viewHeight = viewY2 - viewY;
             uniformBindingManager.setViewPort(viewX, viewY, viewWidth, viewHeight);
             renderer.setViewPort(viewX, viewY, viewWidth, viewHeight);
             renderer.setClipRect(viewX, viewY, viewWidth, viewHeight);