Separate Preview/Export process limits

Most users agree that running two exports at the same time is inefficient and unstable. However, previews and exports are counted equally, and so an export must be completed before a preview can start. Most processors can handle two processing threads at the same time, it’s just not efficient for long term exports. The default setting should be 1 export + 1 preview, to make the best use of the available resources and the characteristics of preview renderings being limited to very short clips. It would make things much more efficient, as while it makes sense to queue exports, previews allow a means of inspiring confidence in settings before adding to the queue, and this system would allow for that while preventing accidentally processing two simultaneous exports.