add a venus fuzzer
The first two commits prepare meson for C++ support. The last commit adds the fuzzer.
The fuzzer is written in C++ so that I can call virgl_renderer_init/virgl_renderer_cleanup just once using a static object. I can switch to C if we don't want C++ in the project. @gerddie
Within <5 seconds of fuzzing, it has discovered several crashing bugs. Time to harden the renderer!