meson: Add top level meson.build

[xclaesse]

It is needed to use libvmaf as Meson subproject, like GStreamer does.

Also fix public header includes:
Applications, and thus our public headers, should be included as
#include <libvmaf/libvmaf.h>. Adding include/libvmaf into include
directories means applications could do e.g. #include <feature.h>
which is ambiguous.

[kylophone]

Seems like this will break everyone's libvmaf build once merged? Is that right? What are the alternatives?

[xclaesse]

Seems like this will break everyone's libvmaf build once merged? Is that right? What are the alternatives?

Yeah, it breaks scripts that does meson setup libvmaf build instead of just make, unfortunately.

I updated this PR to move stuff around so we can build from both toplevel and libvmaf/ subdir. The top level project() call is duplicated as well as meson_options.txt, it's not too bad.

I added a warning() when building in libvmaf/ subdir, that can serve as transition.