STL exports last F6 state, not current file

[sbridger]

Seems that when we are using F5 to update the view, the export STL, that the STL file is from the last time F6 was done.
Should do a new F6 automatically before STL export I think.

[kintel]

Since F6 can take a long time, I'd be more comfortable to rather give the user a warning if he's trying to export a model which has changed since last F6. I quickly looked at it, but some refactoring is warranted to get this to work properly. Since we don't have a GUI test framework, I might not get around to doing it until after the upcoming release.

[brad]

I agree, a warning is the best thing to do.

[MichaelAtOz]

Note in #735 that a zero length .png is produced if F6 is not done.
Issue is wider than just STLs.