JPG/PNG: MuPDF as default provider (#6931)
JPG/PNG: MuPDF as default provider (#6931)

As said in #6929
poire-z/koreader,pazos/koreader,Hzj-jie/koreader,Frenzie/koreader,NiLuJe/koreader,koreader/koreader,NiLuJe/koreader,Frenzie/koreader,poire-z/koreader,Markismus/koreader,koreader/koreader,mwoz123/koreader