Fixes for rendering jittering issues

One of these issues basically undoes the change in https://github.com/HaxeFlixel/flixel/commit/54f5780c335afd1686850a1766c5e3008f62cb63. Rounding was incorrect. It has to be floor otherwise everything rendered in that block will be rounded, while anything rendered outside that block will be floored, leading to jittering all over the place when things have real coordinates.

The other commit is a proposed solution for issues arising from a slowly moving camera causing jittering on things with real coordinates. The idea is to allow cameras to report floored values, as if they are snapping to a grid, thus preventing jittering on anything with real coordinates. The implementation so far works, though it is incomplete, as it does not yet account for the (imperceptible) effect on the algorithms for target following. This would be a matter of changing these to use the realX and realY values of camera.scroll instead of the now floored x and y.

An alternative solution to this might be to make (or allow, or even default to) camera x and y floored when rendering.