FragmentFilter changes and new Bloom Filter

I've created a BloomFilter (like the fog filter in Pixelmator); basically a gaussian blur on a new layer, blended with the lighten mode.

To allow this I've had to make some changes to the FragmentFilter.  One (changing the protection on calculateBounds) is optional but useful if you ever need to do size calculations in a FragmentFilter subclass.

The other is to allow rendering to a filter, rather than the backbuffer; see here for an alternative solution http://forum.starling-framework.org/topic/mist-or-glow-filter

Overall I think this is a better solution as its more de-coupled.  In addition don't thing the (removed) assertion is valid -  Stage3D is quite happy to render AGAL programs to textures.

I think this also resolves #391 (its a bloom filter :); although I'd echo the comments there on making it easier to extend FragmentFilter as, although it's not a major issue given this implementation, it was a pain in the ass while trying other solutions.