Thanks to @EQt's PR #8, greyscale raster images are now handled correctly.
Additionally, the libraries dependencies have been bumped and thanks to usvgs new release, svg2pdf now supports GIF images.
Attention: This release has previously been published as 0.2.1. This was a mistake, because the usvg is exposed through the public API and updating it constituted a breaking change. I am sorry for any inconvenience this may have caused