Merge pull request #36 from houqp/houqp-master

clean up in crengine patch
Merge pull request #37 from houqp/houqp-master

add setVisiblePageCount method for cre.cpp
fix libk2pdfopt that use a different version of libpng
Merge pull request #38 from chrox/master

fix libk2pdfopt that use a different version of libpng