allow cairo to use window instead of framebuffer

[catfact]

for development / test away from norns hardware.

shouldn't be difficult:

1. provide alternative function to create cairo surface from window instead of FB:
   https://github.com/catfact/norns/blob/master/matron/src/hardware/screen.c#L66

(specifically, would probably have to first create the window using Xlib functions, then cairo_create_x11_surface - see https://cairographics.org/Xlib/ )

2. add a preprocessor flag and/or matron argument