General:
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- level file format, loader and saver
- level editor
- sounds and music (using SDL_mixer)
- introduce monsters
- player profiles

Engine:
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- time limit
- define time unit based on engine step

Graphics:
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- menus and HUG (use ftgl?)
- animations
- graphic sets (themes)
- introduce generic model to texture mapping
- special visual effects
- only render currently visible platforms (prevent out of screen drawing)

Elements:
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- arrow platforms: the player can only leave the platform in the direction
pointed by the arrow
- when collected, reverses the player's controls
- when collected, mirrors the view vertically
- when collected, increases/decreases the player's speed temporarily
- lights off / limited vision / clouds / blur
- extra life
- platform that disappears when the player walks over it (with 3-4 steps)
- doors and keys
- teleporter groups, defined by color (4 colors)
- when collected, adds extra time
- jumps
- big hole where the player can fall down to the platform beneath

