compositor preview resolution

Proposal: Have a way to dynamically reduce the size of the rendered image in the compositor. If the render is 10k and the preview value set to 0.1... the working resolution in the compositor would be 1k thus making the working speed much faster. At render/export time, Blender would render at the source res.

This feature exists in the compositor branch here: github.com/m-castilla/blender-compositor-up/tree/compositor-up