Currently we use a bool flag to signal the video thread that it should
call obs_source_deferred_update. This does not work correctly when the
update callback is slow and the update is triggered faster than the
callback can complete.
For example:
* the settings are set to state A
* defer_update is set
* obs_source_deferred_update is called and enters into the callback
* the callback starts making use of the settings in state A
* the settings are set to state B
* defer_update stays set
* the callback finishes
* defer_update is set to false
Now defer_update is false but the callback has only observed settings in
state A but not B.
This commit fixes this bug by keeping an update counter. If the counter
has changed while we were in the callback we know that we need to update
again.
The counter is atomic. The current version uses a plain bool which is a
data race as the value is written and read in parallel.
4508cb03b5