Fixed flickering line/partial screen vibration.
In Rpg and Platformer, the player standing in some places (adjacent to obstacles) caused a black line to flicker across the screen and everything above it to vibrate up and down every few frames.  I think this was caused by the fractional part of the player's velocity getting rounded incorrectly due to using Math.floor when calculating screen positions (I switched it to Math.ceil, and it vibrated in the other direction.)  I switched it to using Math.round to fix that.