Sprite: Change velocity into a tuple

[einarf]

This solves a a few problems

• velocity is now a publicly documented member of Sprite
• Users setting sprite.velocity = 1, 1 will no longer get weird crashes
• Even if the users supplies a list they will be fine

[einarf]

I don't know how this will affect persomance considering we're adding some extra tuple creations.

It can be changed into two separate float value members if needed, but it should definitely not be a list.