Fix underflow issue

It was possible for player position to underflow
after applying negative velocity and platform movement.
Increase player base for colissions

Previously the point of contact between the player
and a platform was 1 pixel width. This caused the
impression that the ball slipped through a platform
when it should have actually collided with it, in
cases where the player just missed the edge of a platform.

Increase this colission width from 1 pixel to a higher,
configurable value to make these scenarios feel more natural.