Add support for non-square heightmap import/export and various related improvements/bugfixes
- Support for non-square heightmap files (.r8, .r16)
- New heightmap import/export dialog
- Fixed issue where imported heightmap is "transposed"
- Fixed issue where saving an r16 heightmap file as r8 or vice versa only changes the extension
BUG:381774
BUG:381830
BUG:381866
Differential Revision: https://phabricator.kde.org/D6460