`resvg` does not isolate background properly

Nobody has claimed this yet.

The following SVG:

```svg

```

Renders fine with default arguments:

but as soon as I add `--background white` I get the following:

I think this is because we just fill the pixmap with the white color and then render the SVG on top of it, with this white color as the initial backdrop. However, I think what should be done is that we render against a transparent backdrop first and then composite the final result onto the white pixmap. This usually doesn't matter for src-over compositing, but if you have SVGs with blending, it does matter in which order you apply the operations.

Start at the CLI handling for `--background` and reproduce the issue with the SVG in the report, comparing default rendering with `--background white`. Trace how the background is applied relative to SVG blending; done means the white background no longer changes the blended result unexpectedly.

Written by the indexing model from the issue text.