arsd.simpledisplay on macos

[Ben Jones]

I'm trying to use Adam's simpledisplay on a mac with XQuartz 
which is installed + running.  When I try to create a window, it 
crashes when calling `XDisplayConnection.get()`.  I'm just 
building dub and have added `"arsd-official:simpledisplay"` as a 
dependency.  Before I installed XQuartz, it wouldn't link because 
of the X related libs missing, so the XQuartz libs did seem to 
get installed to the right place.  The native cocoa 
implementation seemed less well tested than the X version so I 
haven't looked into that too much.

Any idea what's going wrong?  Or is there an example that seems 
to work with the native cocoa version?  I'm only planning to use 
really basic functionality (display an image and maybe capture 
some key presses)

Also, Adam, if you have an idea of what needs to be done to get 
native macos support working, I'm willing to try to give it a 
shot.


```
object.Exception at ../../../.dub/packages/arsd-official-10.3.8/arsd-official/simpledisplay.d(12578): Unable to open X display
----------------
../../../.dub/packages/arsd-official-10.3.8/arsd-official/simpledisplay.d:12578 arsd.simpledisplay.Display* arsd.simpledisplay.XDisplayConnection.get() [0x10f49cc8e]
../../../.dub/packages/arsd-official-10.3.8/arsd-official/simpledisplay.d:13072 void arsd.simpledisplay.SimpleWindow.impl.createWindow(int, int, immutable(char)[], in arsd.simpledisplay.OpenGlOptions, arsd.simpledisplay.SimpleWindow) [0x10f4882ec]
../../../.dub/packages/arsd-official-10.3.8/arsd-official/simpledisplay.d:1688 arsd.simpledisplay.SimpleWindow arsd.simpledisplay.SimpleWindow.__ctor(int, int, immutable(char)[], arsd.simpledisplay.OpenGlOptions, arsd.simpledisplay.Resizability, arsd.simpledisplay.WindowTypes, int, arsd.simpledisplay.SimpleWindow) [0x10f486017]
source/app.d:869 _Dmain [0x10f47dff2]
Program exited with code 1```