Subprojects that use meson.override_dependency() do not require the
caller to provide the dependency variable name inside the subproject.
We also don't want to provide the *subproject* name, because the
subproject name can be `pango-1.50.12` instead of `pango` when using
wrap-file to download the tarball instead of using wrap-git. This
causes the pango subproject to be executed twice when using gtk as
a subproject inside gstreamer (which uses pango-1.50.12 as
a wrap-file).
All the dependencies we use can be switched in this way, but the
remaining ones need to be changed to use meson.override_dependency()
first.
There is no reason to fallback to find_library for cairo and still rely
on pkg-config for all other dependencies, and just when using MSVC. When
building and pkg-config is not working it is preferable to fallback to a
subproject just like for all other dependencies.
Also add cairo.wrap now that meson support has been merged upstream.
