WPE scene wallpapers unavailable on NVIDIA + native Wayland #7

The scene engine needs a desktop GL 3.3 context, but forcing one on native Wayland + NVIDIA leaks a framebuffer-sized allocation per rendered frame (~2 GB/s at 4K — filled 8 GB RSS in ~4 s). Since the fix for that leak, the `gles-api` GDK steering applies only on X11, so on NVIDIA + native Wayland GDK picks GLES and scene wallpapers cannot render — they require an X11-backed mode (`x11-desktop`, which re-execs onto XWayland).

Options: root-cause the NVIDIA EGL desktop-GL leak (driver/GTK upstream?), or render scenes into a separate GL context decoupled from GDK's display context.

See docs/renderers.md "Desktop GL requirement".