FPS Drop when building with all-axis mirrored

[spice basalt]

Reproduce: Build a ship thats fairly well sized using all-axis mirrored.
Cause: Don't know - but I assume something to do with mirror line calculations.

Note that it fixes itself when you exit build mode. (probably a race condition or math being inefficient)

More specific here- it appears to be caused when dragging objects with specific exclusion zones across the ship with mirror mode enabled

*Still checking though

It especially becomes evident when you move the camera around while doing this

Notes: ```

• Corridors, Explosive Charges, Structure Blocks (including wedges), Airlocks, both Crew Quarters, Moving Walkway, Doors and Ion Prisms do not trigger this
• Most internal buildings and objects with exclusion zones trigger this.
• Thrusters seems to trigger this, but to a lesser extent
• Also occurs in planning mode
• This seems to be caused at the boundaries between ships and empty space in the design. (Note that that includes both moving onto the ship from empty space in build mode; and moving from the filled ship to empty space.)```

Note sure if relevant, but if using the corridor fill tool at 10x10 size, small frame skips are noted when moving as per above