Undefine SDL2 DirectFB video driver

[okias]

1. we don't need it
2. it's dropped in SDL 3
3. it breaks compilation on Alpine and postmarketOS with SDL2 enabled (and the bug never going to get fixed on SDL2 side)

[sfan5]

This seems like a strange place to apply that fix.

As far as I understand SDL defines its features at compile time and saves them in SDL_config.h. This configuration is controlled by the SDL package maintainer (Alpine and/or postmarketOS in this case).
So if it is known that DirectFB is broken why not disable it in that place?

[okias]

It's a bit weird place, thou:

• cmake: find DirectFB through a cmake module + use it in installed sdl2 prefix libsdl-org/SDL#6084 (comment)
• https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/29216/diffs

I got inspired by the second link. This removed the support for the Irrlicht, while keeping software really using DirectFB in Alpine having SDL2 support available.

In general it could be patched in Alpine or DirectFB disabled in SDL2 https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/59109

[sfan5]

Personally I would prefer if the the distribution would patch this when needed but also this single line change doesn't hurt and has no downsides.
I'll ask someone else to decide.

[SmallJoker]

My two cents: this line is OK for me but there should be a comment explaining why this is needed and when it can be removed in the future (e.g. forcing SDL3 dependency).