renderer: refactor overlays

Instead of forcing per-image overlays to be rendered at image resolution, allow them to be specified with respect to whatever reference frame the user wants.

Additionally, stop rendering overlays directly onto image frames (even at image resolution), and instead render them onto the final image. This solves various bugs caused by overlays being blended directly onto images that are being processed by hooks, at the cost of reducing overlay scaling to bilinear quality.

TODO:

• test various interactions with rotation