Qt's PNG Image Decoder's default screen gamma value should be 2.2, not 0.0

in src\gui\image\qpnghandler.cpp,

QPngHandlerPrivate::gamma's default value is 0.0, it should be 2.2.

This cause QtWebKit built on Qt when using QT's image decoder incorrectly decode png image with file_gamma=2.50

Url: http://www.schaik.com/pngsuite/pngsuite_gam_png.html