Fix a silly bug in the platformer

- Probably not worth the effort... !
- Resets the multijump flag when on the ground
- Increases the magnitude of a jump after falling from a ledge (without using the jump key)

The bug was as follows:

1. Perform a double-jump onto a high platform, and fall from the ledge.
2. While in the air, press the jump key; the player will not jump.
3. Perform a single-jump onto a high platform, and fall from the ledge.
4. While in the air, press the jump key; the player WILL JUMP.

This patch simply fixes the inconsistency, so that the player can always jump after falling from a ledge.