Unstable API improvements
This MR adds new Meson -Dunstable-apis=true build option that enables unstable APIs by default in cflags of the virglrenderer pkgconfig file by adding -DVIRGL_RENDERER_UNSTABLE_APIS=1 to the common cflags. This was proposed by @gurchetansingh on https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4989346. Crosvm will be able to detect whether unstable APIs are enabled in the installed virglrenderer and use the unstable features based on it.
This MR also stabilizes vtest v3 protocol and makes capsets stable.