Feature Request: export images with a unified DPI of the document resolution

## The Issue
When you are creating complex svgs with many textures and Images, you often end up with a horrible mix of different DPIs and that is really annoying to deal with when printing or exporting to a pdf. In practice you end up either having to manually downscale all the images - really hard to do in a non-destructive way - or export it wiht DPI ranging from your desired resolution to images that take up up to 10~100x the space they really need to.

## The Fix
Add a checkbox for the export that allows you to chose to automatically downscale images to the document DPI when exporting a vector format.

## The Implementation
Put a rasterize node after each image (or raster) source with its footprint set to the image bounds and the resolution set to a fixed DPI for the export.