... which is an asynchronous version of
gimp_drawable_get_sub_preview().

We currently support async preview generation for drawables whose
buffer isn't backed by a GimpTileHandlerValidate tile handler
(i.e., anything other than group layers), since preview generation
fir such drawables may involve processing the corresponding graph,
which isn't thread-safe.

When the GIMP_NO_ASYNC_DRAWABLE_PREVIEWS environment variable is
defined, all drawable previews are synchronously generated.

(cherry picked from commit d79e3fbd)