Some SDL2-* library updates?

[Gianfranco Costamagna]

Hi, in the next few days I would like to work on the various sdl2-* libraries updated.

I'll push the work in an experimental branch and look for api/abi changes on each one.

I'll report back here and maybe push on unstable if the changes and reverse dependencies are safe.

For sure I'll wait for the new sdl2 upload/migration to start the above uploads.

I hope it is ok to do some updates :)
I want sdl2 to possibly go in the next ubuntu LTS, that way we will have more testing before stretch freeze :-)

Cheers

Gianfranco