Bring the oblique slice back to the origin, and bring the translation
gizmo back to its origin a second after it's released, in case it
wanders off.

Textures are owned by the texture-store, which lived until application
destruction. At that point the windows were closed and the
OpenGLContexts were all destroyed. Textures need to be destroyed in the
presence of a OpenGLContext.

Ownership is therefore moved back to MainWindow, and made unique to
MainWindow. MainWindow is destroyed before the child widgets, and so the
OpenGLContext(s) are alive.

This box scaling is quite ugly, and may come back to bite me when
looking at the grid spacing.