HtmlOutputDev: don't use png.h
My mingw build was failing due to png.h not found in HtmlOutputDev.cc.
HtmlOutputDev.cc should not be using png.h. The PNGWriter class is careful to hide the implementations details of libpng.
My mingw build was failing due to png.h not found in HtmlOutputDev.cc.
HtmlOutputDev.cc should not be using png.h. The PNGWriter class is careful to hide the implementations details of libpng.