Screenscaling code.

This is to fix some problems with screen scaling, and to add more options to screen scaling.

In the old code resizing the window can effect HXP width and height.
The old HXP.resize()  also didn't seem to work very well in my experience.

This allows for different screen scaling modes to be easily used by changing the new HXP.screenscaleMode.
Options:
Force - Scales disregarding aspect ratio. How the scale functioned functioned previously
AspectRatio - Scales with aspect ratio. The default setting.
Multiples - Scales by multiples of screenscaleMultiples. For example use to keep pixels scaled evenly.
Custom - Use screenscale variables and set the scale yourself.
Setsize - The window sets the game width and height and scale is applied from screenscale.

The code also places the game screen in the center of the window.

I wasn't sure where I should put the new variables in the code so I just put them above the resize function.