Move driver graphics out of main package

[andydotxyz]

Remove the pixelgl import from the main package.
This enables alternative graphical drivers.

[HFO4]

I remember pixelgl requires to take over the main function.🤔 I
will check this in few days.

[andydotxyz]

Yes, this patch does not change the thread it runs on, it just launches the pixelgl only if required. It moves the pixelgl runner to later in the code but still on the main thread. I tested this with an alternative (Fyne) graphics system that also requires starting from the main thread.