Increase horizontal movement speed while jumping

Some jumps (particularly in the Dwarven level) were excessively hard in
Arx Libertatis. Somewhere along the way we managed to decrease the jump
distance compared to vanilla Arx Fatalis - this is supposed to fix that.

There are still problems where the jump height is decreases with lower
framerates, all jumps I tested where doable at 15fps and higher.

Fixes: bug #413