fixes for linux

[npisanti]

i had to modify some classes as the compiler wasn't building the addon, complaining about those non-void function not returning a value. Tested on Debian 9, with clang-6.

Thanks for you work on liquidfun!

[ghost]

I was about to make exactly the same pull request. I think this would not be only for linux.

[eaviles]

I was having the same errors on Mac OS Sonoma 14.2.1, on an M1 Pro. These changes fix the build on my app on version 0.11 of openFrameworks.

[eaviles]

@tado could we merge this?