platform_s should act like other platforms

[Jigoku]

Originally when the platform_s entity was added, it was intended to be a platform that would allow you to walk left/right whilst it was swinging, but there were problems with collisions/gravity at the time.

The current implementation with the "cradle" that catches the player is an ugly hack which just stops the player from moving left/right (requiring a jump to exit) and sets a random Y velocity.

Original implementation looked like this, https://www.youtube.com/watch?v=APUnwhrV57o but with the obvious problem of the player bouncing all over the place at the time.

[Jigoku]

fixed in #114