Qt imageformat plugin for Webp

It's forked from cor3ntin/qt_webp.

####Building:

1. copy all the source code files to %QTDIR%\src\plugins\imageformats\webp
2. go to libwebp folder, run "nmake Makefile.vc CFG=release-static"
3. back to up folder, run "qmake -tp vc"
4. build the qwebp.vcproj

####Using:

make sure that the qwebp4.dll is in the Qt plugin folder. any image files with .webp ext will work fine with QPixmap QImage

#####Reading:

QPixmap p(":/images/demo.webp");

#####Writing:

QImage p(":/images/demo.png");
//quality >= 100 means lossless
p.save("c:/temp/demo.webp", "webp", 100);

#####Known Issues: There are some webp source code file in enc and dec folder with same file name but different contents. I have no idea to spec obj file name for each source file, the obj files will overwrites each other, so I can't integrate webp code to qt project.

Steven Yao