Since QImage does sanity checking for overflows and stuff wrt.
dimensions and depth, check for QImage::isNull() as early as possible to
see if there's some funky business going on.
Also tried to add some checks wherever we wrote to "raw" memory.
Unit tests pass, and tested converting some files from
https://samples.ffmpeg.org/image-samples/ to pngs, and that seemed to
work.
Reviewed By: aacid
Differential Revision: https://phabricator.kde.org/D24367
Frameworks have a convention of naming uninstalled headers in src/ with
a _p at the end of the name, to make it clear they are not part of the
API. None of the headers in KImageFormats are installed, so it is not
really necessary to follow this convention, but we follow it anyway for
the benefit of both humans and tools (like kapidox).
