Use gimp_tile_handler_validate_validate(), added in the commit
before last, in gimp:buffer-source-validate, in order to pre-render
the necessary region of the buffer, instead of performing the
validation implicitly by iterating over the region.  This is both
simpler, and, more importantly, allows us to render the entire
region in a single chunk, instead of tile-by-tile, which can be
considerably more efficient, especially with high thread counts.

This essentially extends the dynamic sizing of rendered projection
chunks to layer groups, which are rendered through
gimp:buffer-source-validate, rather than just the main image
projection.

(cherry picked from commit 83dd94ba)