main/postmarketos-base-wayland: tell SDL applications to run in Wayland mode

Currently, SDL applications default to running in X11 mode, even when the graphical environment you use uses a Wayland compositor. This is not ideal as in e.g. Phosh X11 applications are generally blurry and don't fit the screen very well. SDL2 applications work significantly better in Wayland mode in Phosh than in X11 mode. To run SDL2 applications in Wayland mode, one has to set SDL_VIDEODRIVER=wayland.

However, this is:

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

And as such cannot be done right now. Reasoning can be found in the linked issue.