Multi-thread tile build blending

Apologies if this has already been requested or if I’m doing something wrong.

When I do a tiled build of many tiles (eg. 16x16 tiles) each tile is rendered using all the threads available and in parallel. Only once all the tiles are done a single thread does the job of blending and this can make the overall time to create a set of tiles very long, especially when I do relatively simple renders but across a lot of tiles.

In my example, 256 tiles of 512x512 each can be generated in ~30 seconds, but then takes almost 10 minutes to blend the tiles one by one.