Meson: Various fixes and simplifications

This fix usage of gegl as subproject. Bump required version to 0.54.0, it is needed for meson.override_dependency().

Since all gegl dependencies are themself using Meson build system (libjpeg and libpng from wrapdb), they can all be built as subprojects. This simplifies a lot the build on platforms that does not have a package manager, like Windows, or when cross building.

Note that babl.wrap uses git master branch instead of a release tarball because it needs babl!48 (closed) (not merged yet).