Add a background texture to the window

Depending on the window size or aspect ratio, there would be black empty areas around the game screen. This space is now filled with something more interesting!

Other minor changes are included:

• Use integer scaling (2x, 3x, 4x..) to avoid unevenly sized pixels.
• Enforce a reasonable minimum window size.
• The game "screen" is now centered properly in the window.
• In menus, the ESCAPE key (and pause button on SMS) can now be used to go back/cancel.
• Hotspots for using the menus with the mouse are now more accurate.