Down sampling in post-process filter changes viewport size

Ran into this issue with BloomFilter, not tested with other filters.

When setting down sampling for the filter with the setDownSamplingFactor(), the 
size of the viewport gets changes. With a value of 2, only one quarter of the 
actual window size gets rendered to (width and height / 2).

To reproduce:
* Create new "BasicGame" project.
* below "rootNode.attachChild(geom):", add:

FilterPostProcessor processor = new FilterPostProcessor(assetManager);
viewPort.addProcessor(processor);
BloomFilter bloom = new BloomFilter();
bloom.setDownSamplingFactor(2);
processor.addFilter(bloom);

* start the project with default settings
* The cube will be visible in the bottom left of the screen, instead of the 
center.

As far as I can tell this was introduced in r10750 
https://code.google.com/p/jmonkeyengine/source/detail?r=10750

Original issue reported on code.google.com by the.eisb...@gmail.com on 2 Sep 2013 at 7:08

Implorer / jmonkeyengine

Down sampling in post-process filter changes viewport size #599