The existing behaviour traces back from pre-2.5 era. It seems like a
general sanitization technique for any operator which may change the
view transformation.
While it still makes sense to switch to PERSPECTIVE mode when actively
calling a view operator, I don't think the same should be said by simply
switching the active scene.
This PR makes it so a temporary flag is set when switching to a scene that
has no camera. This flag is kept around until we deliberately change
perspective mode. We can eventually make it so the flag is reset again in
more occasions. But the patch is good as it is.
Ref: !146364
e8deba14c3