Change zoom limits for very large machines

[euphy]

Also change the default zoom level so it makes the machine fill the window on startup. At the moment it is set to 100% which is pixels == mm. Fairly arbitrary.

[euphy]

Default "initial" zoom level now fills the window vertically.

[euphy]

Sometimes this backfires - it looks like the sequence that the code runs in isn't fixed, and sometimes the zoom level is set before the window has finished being sized.