Prototype SVG to PNG conversion tool using Qt WebEngine

Based on the discussion in QTBUG-109067, a build-time tool to convert SVGs to PNGs would solve the most immediate need.

SVG images could be rendered into PNGs for multiple resolutions (image.png, image@2x.png, image@3x.png, image@4x.png...) at compile time. The PNGs could be added as resources with a given prefix.

The CLI tool would have options such as:

• which resolutions to generate pixmaps for
• which resource prefix to use (or perhaps whether to embed resulting pixmaps as resources at all, although without that it might become very cumbersome to write reliable code that loads them).
• perhaps image format (PNG as default might be enough; no value in supporting lossy formats, perhaps?)