Bump Weston's Meson requirement
We have quite some things waiting for a Meson version requirement bump:
- #180 wants Meson 0.50
- #269 but does not seem resolved in Meson yet: https://github.com/mesonbuild/meson/issues/4011
- #293 would want Meson 0.49
- #289 might benefit from Meson 0.50
- #295 needs Meson 0.50.1
Could we bump the requirement and how far?
If we go for 0.50 or later, I have a vague recollection that we should pick a 0.x.1 at least for some fixes.