main/postmarketos-base-ui: run SDL in Wayland mode by default

This is possible now that all SDL (1) applications use sdl12-compat instead of "classic" SDL 1.2. Note that this only is applied to user interfaces that use Wayland and are launched through tinydm.

See https://gitlab.alpinelinux.org/alpine/aports/-/issues/12739

Draft because testing/ still remains to be adapted.