further dependency updates

Hi,

I had some closer look at the dependencies and saw that the ebuild had some missing dependencies according to the official github page [1]. I've also crosschecked them with my local installation.
Please have a look at my changes. Sadly it got a bit more complex. I've also reworked that part with the ffmpeg/libav use-flags. What do you think about my solution?
I also removed the "die" for emake. Since EAPI 4 those die's for ebuild function's are not needed anymore. I especially checked for emake at the gentoo devmanual :)

[1] https://github.com/MaartenBaert/ssr

On a different note:
I got glinject working on my system (with the multilib builds). Really cool. :) You can check for details here: https://github.com/MaartenBaert/ssr/issues/66

Regards,
mike