macOS: Calculation of velocity returns values near 0

[chriz-zeller]

This issue is generated to split #68 in which two problems were found while debugging.

The calculation of the velocity in Game.hs (line 519) returns invalid and very small values near 0.

@asandroq:

1. Can you figure out, which part of the calculation makes the number nearly zero? That is, what are the values for refPosPaddle ci and p in each cycle?

2. Besides, if I remember correctly your paddle is only for a very short moment in the middle of the screen (initPosPaddle) and then it immeadiatly goes to the left side of the screen. If you do not move the mouse, I think, it should stay in the middle of the screen (initPosPaddle = ((gameWidth - paddleWidth)/2, yPosPaddle)). Can you take a look what are the position values at the very first steps of the game.

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Reminder for further consideration: In issue #68 the events LostFocus and GetFocus are mentioned. Maybe at a later time we should take a look whether this is related to this issue.