Linux support for tsound (SDL2), from pr #51

[ideasman42]

Credit goes to François Revol for this work.

[kogaki]

Jenkins, confirm this

[kogaki]

SDL2 is under zlib license. So it seems ok to include them in thirdparty directory (if needed). https://www.libsdl.org/license.php

[shun-iwasawa]

Implement with SDL seems to be a reasonable solution. LGTM

[shun-iwasawa]

@ideasman42
Thank you for a lot of improvements you have made to the software.
You have brought consistent steps for realizing the Linux build, and moreover great benefits for other OS versions.
We'll be happy if you could contribute OpenToonz continuously and would like to assign you as a committer.
Let's improve OpenToonz together :-)

[ideasman42]

Thanks @shun-iwasawa, I'll still do PR's for anything I'm unsure of.