Update SDL to 2.0.6 about berserkerquake2

[UCyborg]

It seems newer SDL versions favor WASAPI as the default sound backend on Windows, resulting in sound initialization failure. If they're not planning to drop DirectSound support as well, adding a console variable to set sound backend and defaulting to DirectSound might restore the sound without having to manually set environment variable. Otherwise, sound code would have to be updated. I might look into this when I have more time.

[ROCKNROLLKID]

2.0.7 is now the latest, but nothing new on he sound backend. I don't think they have any plans to drop direct sound, as far as I am concerned.

[hgdagon]

@UCyborg since we're talking about sound, let me mention that I had a problem with all SDL apps on my current (Win10) laptop. I did not have that problem on my Win7 laptop, but, apparently, you need to define an environment variable for that to work. Surprisingly, I never defined that on my Win7 and all SDL apps (included those that failed on Win10) worked just fine...

[UCyborg]

It's possible to get the game to use WASAPI without altering the code passing the sound samples, so just calling SDL_OpenAudio with NULL obtained parameter, which makes SDL2 do the necessary conversions. Apparently passing audio to WASAPI can only be done using 32-bit float format, while the old sound code uses 16-bit integer format. I tried that, but it makes it sound kinda odd...

@hgdagon There's no sound in Berserker by default neither on Windows 7 under the same conditions with SDL 2.0.6 and above. Setting environment variable via batch file is one way, but it can also be set programmatically.