Rejiggered the blender handling: allow up to N blenders active at once so
that with small numbers of avatars, the blenders can stack for lower latency.
Merge branch 'master' of https://github.com/highfidelity/hifi
Merge pull request #3443 from ey6es/master

Allow blenders to stack up to a point in order to reduce latency with fewer avatars (but still cap work with lots of avatars).