smpeg 0.4.5

I don't know much about the history (#6), but the smpeg repository now seems to have [stable versions tagged](http://svn.icculus.org/smpeg/tags/).

Here is an updated formula, which hopefully pull itself back to the core (#68):

1. `url` now points to the version tag of 0.4.5. It is the latest version in 0.x release.
2. `head` is now dropped. Instead, I've created a separate formula for `smpeg2` dependent on `sdl2`. It will be posted in the the main tap.
3. `gtk+` dependency is now optional. I don't think people really wants to use this sample player. `smpeg` is more likely used as a library.