Bump base for [fix] MuPDF Android font dir patch update (#5352)
Bump base for [fix] MuPDF Android font dir patch update (#5352)

https://github.com/koreader/koreader-base/pull/971

Fixes <https://github.com/koreader/koreader/issues/5347#issuecomment-529480686>.

Also includes:
* [feat] Add FFI RTC interface (<https://github.com/koreader/koreader-base/pull/969>)
* Bump thirdparty/djvulibre to current master (<https://github.com/koreader/koreader-base/pull/970>)
poire-z/koreader,Frenzie/koreader,Frenzie/koreader,mihailim/koreader,pazos/koreader,Markismus/koreader,koreader/koreader,koreader/koreader,mwoz123/koreader,Hzj-jie/koreader,poire-z/koreader,NiLuJe/koreader,NiLuJe/koreader