At least on GCC on Linux, it appears std::function has noticeable
overhead compared to blender::FunctionRef. That makes some
sense, as the latter generally handles less, and the performance
difference is mentioned in the function ref header as well.
To test performance, I measured the timing of the first data
upload (`BKE_pbvh_draw_cb`) after entering sculpt mode. For
meshes, I observed a 30% improvement, from 1.7s to 1.3s.
For multires, I observed a change from 290ms to 263ms.
The change should apply to regular draw updates while sculpting,
but that's harder to measure.
This is also cleaner semantically, since the callbacks aren't meant
to own any data, they are just lambdas that capture by reference.
25917f0165