Parameterizing height/width in ai2thor-xorg start

[Lucaweihs]

Allowing for changing the height/width of the x display when running ai2thor-xorg start by using the --height and --width flags.

[mattdeitke]

A related question here, are things slower if height and width are increased dramatically - e.g.,

DEFAULT_HEIGHT = 2500
DEFAULT_WIDTH = 2500

And is there any good reason for the defaults being fairly low? It seems many people have run into this issue, where the frames don't render fully because the height/width is too small.

[Lucaweihs]

I haven't had any chance to experiment with what the FPS impact of changing the display is, I would guess it's minimal but until we get some concrete numbers I'll keep it low for now.