Use Math.round in drawing functions instead of Math.floor (or its shortcut: ~~)

[parasyte]

There's a small issue with sub-pixel gravity and Math.floor that can cause "bounciness" on a single pixel. See for an example: #538 (comment)

The fix is simple; use Math.round instead. This ticket is here to start a discussion around whether we want to start standardizing on using Math.round in places where we've used the inversion shortcut with double-tilde: ~~. I'm for using Math.round, and it's probably even easier for JIT engines to optimize than the double-inversion operations.

[obiot]

I'm ok with it, as I was saying in the other ticket, yes the performances with ~~ is faster... but with the performance improvements brought by the quadtree, I suppose that we could lose a tiny bit of speed here in exchange for proper drawing position :)

[parasyte]

Looks like you're right about performance: http://jsperf.com/math-round-vs-double-tilde

So we'll just add 0.5 before the double-inversion. ☺️

[obiot]

Huh... I never thought about that trick :p
Some other places in melonjs could probably also benefit from it

[parasyte]

@obiot I'll close this one, since it resolves the bug as reported. I also made an earlier change to replace Math.round with the same trick. There might be other areas in drawing where sub-pixel positions cause bounciness, but I think Entity is the biggest one.