Fullscreen, post-processing, parenting bugfix

* Fullscreen mode was added. The canvas size stays the same to provide a
consistent viewport (rendering result is only zoomed).
* A post-processing framework was implemented. Filters can be added any
time during the rendering, they will be applied to the final image when
everything is done. Currently 1 finished filter: HSBAdjustFilter (change
Hue, Saturation, Brightness). WARNING: Can slow down your game a lot!
* A bug with parent entities was fixed. Previously, when entities were
removed from the game, they were still saved by their parents. This
killed the FPS count in very little time. Noticable with projectiles,
for example.