Textures can be filtered, +RotateFilter

* The Texture class received the method filter(Filter, boolean), which
makes it possible to generate new textures by applying a standard
post-processing filter to an existing texture.
* Two shortcut methods were added, for rotating and flipping the
textures.
* A RotateFilter was added for, of course, rotating textures (and also
the screen if wanted, which probably makes for some awesome camera
shaking effects).