Fix exporting ORA files (https://bugs.kde.org/show_bug.cgi?id=423088)

I am sorry, that I am still not sure of the best format you would like the commit messages / branchnames in for bug fixes, is there a reference for this somewhere? Anyway I have tried a few examples including the last one in the KA thread here: https://krita-artists.org/t/bug-exporting-to-ora-produces-blank-file/8431/12 , and I was seemingly able to fix the issue with this method.

While gathering the x + y offsets, and bounds for saved layers in ORA, use the layer->exactBounds() method instead of the plain layer->x() and layer->y(). This seems to correctly capture transformations outside of the original layer bounds correctly.