Add 'threads' option to ffmpeg invocations

[jesusbagpuss]

By default, ffmpeg will be 'greedy' in it's CPU use.

This can be limited by defining a -threads X option to the invocation.
The value of 'X' should be chosen appropriately for the resources on the server - e.g. 50% of the number of CPUs - to leave overhead for web requests to be handled.

Lowering the number of threads may mean ffmpeg calls take longer to complete - but the benefit of leaving resource for Apache outweighs this.

There are a few ways to address this (and possibly more):

• document the use of the -threads X option on the EP-wiki. The invocations can be set in repo config, so if someone wanted to use the threads option, they could add an appropriate option to their archive config.
• hardcode it in ~/lib/syscfg.d/invocations.pl
• add a global config option, defaulting to zero (to maintain existing way it works). Would need to be interpolated in ~/lib/syscfg.d/invocations.pl. Option can be overwritten in archive config.
• add to Plugin::Convert::Thumbnails as e.g. $DEFAULT{ffmpeg_threads} = 0;. This would allow most tuning of the option.

If an appropriate approach can be decided, I'll do the coding.