An ammendment to previous fix, and a slow-camera jittering fix. closes #797, closes #1122

Along with some general cleanup in the areas I touched, this PR mainly does two things:

* This reverts the rounding of sprite rendering when FLX_RENDER_BLIT and not simpleRender. This does not unfix any of the issues my previous update fixed, but prevents flash sprites from blurring and jittering when they have an odd size and are complex rendered.
* I replaced repetition of the FlxObject#getScreenXY() implementation with a call to the function. This has made it possible to do consistent rounding between rendering and collision checking, and in turn, this fixes the jittering issue in https://github.com/HaxeFlixel/flixel/issues/797. The `pixelPerfectPosition` flag now indicates that positions should be rounded **prior** to adding the cameras position.

Somebody may wish to implement something along the lines of the `pixelPerfectRender` property on `FlxCamera` to allow users to set a default `pixelPerfectPosition`. Not sure how useful that would be, though.
Why is this necessary?