Update: 4 players, gamepad support

What's new:

• very basic gamepad support (use left analog stick or D-pad to move, press "A" to jump, "X" to place objects)
• 4 player support: 2 of them are controlled via keyboard, 2 via gamepad
• internal changes to player graphics (player objects are greyscale, color overlay is applied for each at initialization / respawn)