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.

Start by tracing the vector export pipeline and the rasterize node described in the issue. Determine how export options expose document DPI and how image or raster sources can receive a bounded rasterization step. Done means an export checkbox applies the document DPI to those sources when exporting a vector format, without changing the source images.

索引モデルが issue の本文から書いたものです。