GDI driver: managed to coax out a working find_primary_display_adapter function
GDI display driver: explicitly use DISPLAY_DEVICEW rather than DISPLAY_DEVICE.
GDI driver: removed find_primary_display_adapter() again. I'm not gonna spend any more time on this.
gdi display driver: basic, simplified fullscreen support (window creation only)
GDI display driver: fail if fullscreen window cannot be created.
GDI display driver: added low-tech display mode changing code for bnz.

This is sufficient. The OpenGL display driver is using GLFW and will get something more sophisticated.
GDI display driver: don't scale image in fullscreen mode. Center it instead.