SDL2 Tutorial update

SDL2 is now up to ver 2.0.7 release and 2.0.8 dev. The Raspbian Stretch changes we had to patch the source for have been included into the official source no so I’ve updated the tutorial to fix the process. I’ve also updated the utility library versions to the latest ones and removed the smpeg util library. It was only included because it was a requirement for mp3 support in the mixer library and that has been replaced by mpg123 in the latest version. The smpeg library hasn’t been updated since 2013 and no longer compiles clean.

SDL2 2.0.8 on Raspberry Pi