[fix] MuPDF: set default DPI-based EPUB fontsize (#4372)
[fix] MuPDF: set default DPI-based EPUB fontsize (#4372)

Makes an attempt at a reasonable default. References #4368.
NiLuJe/koreader,Frenzie/koreader,pazos/koreader,koreader/koreader,koreader/koreader,houqp/koreader,NiLuJe/koreader,Markismus/koreader,Hzj-jie/koreader,Frenzie/koreader,poire-z/koreader,mihailim/koreader,mwoz123/koreader,poire-z/koreader