Export Panel should allow you to set frame size by percentage

If I want to send multiple sequences to the media encoder at, for example, half-resolution there is currently no way to do this. This becomes a bottleneck in particular when I have a number of sequences at different aspect ratios, and there should be a simple way to apply this 50% downscale to all of them, but instead I need to go through them one by one, or send to media encoder in batches. This extra time and effort really adds up for complex deliverables.