Draft: d/tests/gpu-console: Explicitly use offscreen video driver

In a CI environment we can't expect to have a working X11 display or Wayland compositor unless we explicitly run one, which this test does not. In "classic" SDL2 this was masked by the offscreen video driver being enabled by default, but in SDL3 + sdl2-compat the offscreen video driver is only used if it is specifically requested, with the reasonable justification that video drivers that don't show any pixels to the user are only rarely what is wanted.

Closes: a bug whose number I have not yet received