Meshes that require adaptive subdivision are currently tesselated one at
a time. Change this part of device update to be done in parallel.
To remove the possibility of the status message going backwards, a mutex
was required to keep that portion of the loop atomic.
Results for the loop in question: On one particular scene with over 300
meshes requiring tesselation, the update time drops from ~16 seconds to
~3 seconds. The attached synthetic test drops from ~9 seconds down to ~1
second.
Pull Request: https://projects.blender.org/blender/blender/pulls/145220
