Add wxPNGImageData and use it to store variables that used to be local
in wxPNGHandler::LoadFile() and could be clobbered by the use of
longjmp() and pass these variables to DoLoadPNGFile(), which still uses
longjmp(), whereas LoadFile() doesn't any longer.
In addition to fixing the warning, this allows to use C++ dtors for
freeing memory and makes the code shorter and, arguably, more clear than
the original version (although written in an unusual way) and definitely
more clear than the version with the warning fix.
This reverts commit 99ea06203a ("Fix
clobbering warning") and replaces it with a different solution to the
same problem.
e24e9323e0