[ft2] should we remove the `r_font_use_alphatexture`? It’s broken for a long time

[NaitLee]

As the developer of #67 I had to deal with the r_font_use_alphatexture thing appeared in ft2.c. Workarounds like this are considered (before testing and finding out this true issue) but didn’t rescue this fact —

After searching around I’ve found that this commit in master cause the configuration r_font_use_alphatexture 1 to segfault the game, and probably also caused #66.

That commit is from 2018 and so far no relative hassles have been reported, that further explains that this “feature” isn’t used.

For the ease of code maintenance (and #67) it’s suggested to remove it just like #66.

Other notes:

• r_font_use_alphatexture 1 can work in div0-stable branch for not including this commit, but it shows only black text (rather than white & shadowed). You can see it easily by launching xonotic 0.8.5 with this as parameter:
  $ ./xonotic-linux64-sdl +r_font_use_alphatexture 1
• With the commit before that (5a126af930cb0f3c66b3065218d939d4bbacb613), the game can start with r_font_use_alphatexture 1 but glyphs are all white squares, also considered broken
• You may git checkout to these branches/commits and compile to test all these out

[bones-was-here]

Probably it worked when implemented, but I expect the newer font features and more generic rendering paths aren't compatible. For saving font memory in VRAM there's r_font_compress which still works.

[bones-was-here]

I removed the last bits f0f6982297b24bf60c35503f705a11d83a549cfe