Implement edge scroll for game camera

[joshuaboniface]

Prefix: Absolutely love this, thank you for the work! DK2 is one of my all-time favourite games, I just started getting back into it, but the sheer bugginess of the game makes it really frustrating. I can't help much with the code, but I can certainly test!

Platform: Linux x64 (Debian 10)

Issue: Moving the mouse to the edge of the screen does not move the camera.

Logs: Nothing stands out, but I get these warnings around the time I tried moving then dug out some tiles:

Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Doors/steel_door.png] uploaded to atlas (atlas texture id: 1).                                  
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Doors/magic_door.png] uploaded to atlas (atlas texture id: 1).                                     
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Doors/secret_door.png] uploaded to atlas (atlas texture id: 1).                                    
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/sentry.png] uploaded to atlas (atlas texture id: 1).                                 
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/Guard_spot.png] uploaded to atlas (atlas texture id: 1).                                  
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/fear.png] uploaded to atlas (atlas texture id: 1).                                           
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/alarm.png] uploaded to atlas (atlas texture id: 1).                                          
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/gas.png] uploaded to atlas (atlas texture id: 1).                                    
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/spike.png] uploaded to atlas (atlas texture id: 1).                                       
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/trigger.png] uploaded to atlas (atlas texture id: 1).                                        
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/boulder.png] uploaded to atlas (atlas texture id: 1).                                        
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas                       
INFO: Image [Textures/GUI/Traps/freeze.png] uploaded to atlas (atlas texture id: 1).                                 
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas           
INFO: Image [Textures/GUI/Traps/lightning.png] uploaded to atlas (atlas texture id: 1).                                                                                                                                                       
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas                     
INFO: Image [Textures/GUI/Traps/fireburst.png] uploaded to atlas (atlas texture id: 1).                                      
Apr 28, 2020 1:10:09 AM de.lessvoid.nifty.render.batch.BatchRenderImage uploadImageToAtlas
INFO: Image [Textures/GUI/Traps/Jackinthebox.png] uploaded to atlas (atlas texture id: 1).                                   
Apr 28, 2020 1:10:10 AM toniarts.openkeeper.world.animation.AnimationLoader playAnimation                                                                                                                                                     
SEVERE: Creature animation playing error: java.lang.NullPointerException                                                                                                                                                                      
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask                 
INFO: Added task Dig tile at java.awt.Point[x=9,y=21] for player 3!                                                  
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask                 
WARNING: Already a task Dig tile at java.awt.Point[x=9,y=21] for player 3!                                           
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask                                        
INFO: Added task Dig tile at java.awt.Point[x=9,y=22] for player 3!                                                  
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask                            
WARNING: Already a task Dig tile at java.awt.Point[x=9,y=22] for player 3!                                                  
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask                                      
WARNING: Already a task Dig tile at java.awt.Point[x=9,y=22] for player 3!            
Apr 28, 2020 1:10:20 AM toniarts.openkeeper.game.task.TaskManager addTask         
WARNING: Already a task Dig tile at java.awt.Point[x=9,y=21] for player 3!