Update SDL_TTF to latest upstream (and maybe get rid of libraqm)

[tobbi]

SDL_TTF didn't have support for Arabic until a few years ago. Thus, we included libraqm to help with proper text display.

However, since a few years back, SDL_TTF has gained support for Arabic text shaping, see libsdl-org/SDL_ttf#66 (comment)

We should check the commits made by us on our SDL_TTF submodule, see if they're still needed and then maybe rebase them against latest SDL_ttf 2.0 upstream.

libsdl-org/SDL_ttf#66 (comment) mentions the steps required for getting Arabic to work.

We also need to make sure that SDL_TTF is compiled with harfbuzz support.

[tobbi]

What confuses me is that our latest "supposedly upstream" commit is not actually on upstream: I can't find SuperTux/SDL_ttf@0617926 anywhere else, not under that commit hash, nor using the title.

EDIT:
I found libsdl-org/SDL_ttf@dfcea82 which is the "Update copyright year to 2018" one. We have this commit on our repo as well, as SuperTux/SDL_ttf@55bb1f5

[tobbi]

Anyway, @MatusGuy mentioned on IRC that current SDL_ttf does not have bi-di support as of today, so using it as a drop-in replacement likely won't work.

I might still consider adding a flag to CMake to use the system SDL_ttf, though.