Add webp support to Windows deps (and other fixes)

A pile of untargeted fixes not worth their own MR. Ignoring the refactoring / general maintenance / obvious fixes, the consequential ones are:

• Add webp-pixbuf-loader to msys2installdeps.sh

  Needed to make progress towards #5089. Doesn't need an immediate Windows CI update, but would be helpful to confirm the changes are correct. @dehesselle

Additionally, in

• Move symbol_convert() out of libUEMF

  @doctormo might want to confirm the file movement is to the right place, and the licensing is correct (see the commit message).