Filter updates, +BlurFilter, ticking fix

This update includes many updates to the post-processing ("filter")
system. These include:
* A BlurFilter was added, which does what the name suggests - it blurs
the rendered image.
* Instead of letting each filter decide individually, you can now force
them to make a copy of the input image before modifying it. By default
filters will try to avoid this though, solely because of performance
reasons.
* Filters can tell the Display (or whichever object calls them) that
they do not need to be run on an image, perhaps because the filter's
properties are set to redundant values. This saves a lot of computing
power in these cases and makes the game run faster.
* Methods were added to simplify cloning an image, as well as creating
an image that has the same properties but is empty. (see
Filter.cloneImage() & Filter.newImage())
* Because of the other changes, the ShapeRenderer now supports being
transferred over to a different Graphics2D object.

Also, a ticking bug was fixed, where the game would catch up on lost
ticks during the init() phase, which resulted in the game jumping to a
point in the future, a point where it should not be yet. This would be
especially problematic when making a game that starts immediately after
launch, without showing a menu first.