Rather than having the whole Win32 cross-build into the 'gimp' stage,
break the dependencies and GIMP-only builds in 2 stages.

Since apparently we need to keep the same structure for the native and
cross build (otherwise we don't get parallel builds; in other words, I
didn't find the possibility to set separate pipelines up), I move babl
and GEGL into the same 'dependencies' stage.

Finally I remove the -base rules extended into actual jobs, except for
`.gimp-base` (this is the only which makes sense as it is actually
common to the meson and autotools build).