Fasttracker II clone v1.56

• Fixed some possible issues with fullscreen in Linux (still buggy)
• Fixed some issues with restoring the window size and image stretch mode when loading or resetting the config when the program is running
• Bugfix: Focus was lost when drag-n-dropping a file onto the tracker, which meant that the first following mouse click was ignored
• Windows/macOS: Updated SDL from 2.0.20 to 2.0.22