The GIMP image loader had a limit to 16K x 16K pixels, because this would
already exhaust the 2 GByte address space limit of 32 bit systems.
Remove this limit on 64 bit systems to allow the full 32K x 32K size.
BUG: 391970
Differential Revision: https://phabricator.kde.org/D12557
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).
