Use gimp:buffer-source-validate, introduced in the previous commit,
for the source node of GimpDrawables.  This avoids threading issues
with layer groups, or any other drawables that may use a validating
buffer, by making sure the buffer is validated before any
succeeding operations, and hence the associated graph is processed
on the same thread as the parent composition.

Restore multithreaded processing in GimpOperationLayerMode.