Add Meson build and Gitlab CI

Hi all,

This is to add Meson and CI support for libvpx. As you can see, there is one compulsory cherry-pick to make it work for Surfaces.

Apart from that, I've copied the CI file from FFmpeg, and adapted it for use here. I've had to remove references to yasm, though, because of Meson passing Nasm-style optimization and debugging flags to Yasm, thus breaking the build: https://github.com/mesonbuild/meson/issues/11726

Please let me know what you think!