https://forums.uberent.com/threads/wip-papa-texture-editor-v0-4-1.56309/ Works especially well for sky boxes. Sort of tutorial here: https://forums.uberent.com/threads/...engine-and-import-it-to-pa.56567/#post-873085
Eh? Seems to work fine for me. Direct link is https://github.com/DeathByDenim/papatextureeditor/releases The .bin is for Linux, the .dmg is for OS X, and the _win.zip is for Windows.