Allow overriding QBmpHandler hard limit on .bmp dimensions

QBmpHandler returns false when loading images greater than １６３８４ｘ１６３８４ preventing easy loading of large bmp files. The limit seems to have been introduced with:

https://code.qt.io/cgit/qt/qtbase.git/commit/src/gui/image/qbmphandler.cpp?h=5.9&id=5104a529ce5aea5e94101770ece188b98f20baaa

Suggestion: make the limit based on image file size rather than dimensions and/or allow the size limit to be overridden.