I had added an optimization to not pass messages to
main thread every time the CVDisplayLink timer fired, unless
a render frame for that monitor was actually requested.
However, this optimization is impossible to implement wihtout a deadlock
since CVDisplayLink has its own internal lock that it does not expose.
So I guess macOS users with multiple monitors will simply have to take
the performance hit of useless wakeups sixty times a second for every
extra monitor.
Fixes#1779
39f6071c68