app: use gimp_tile_handler_validate_validate() in gimp:buffer-source-validate
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.
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