[Instances] Instances index overflowing to second continent

[RexTheWho]

Describe the bug Diesel engine issue but lack of index settings mid-editor can quickly cause this.

When using a massive amount of instances (100-200 range w/ index size of 500) it is very likely you will reserve enough spaces that instance units overflow into the next continent. (World = 100000, Editor_Only 200000)

To Reproduce Steps to reproduce the behavior:

• Make a new level
• Spawn lots of instances with high index sizes
• World: Select All on the Editor Only continent and you should see some selections over instance units

[RexTheWho]

Poking around at some stuff. Editor seems to never clean up the start_index's in the continent so if you delete lots of instances you end up with wastes spaces, or 'index bubbles'.

If you could press a button to reorganize the start_index's of instances that might help with this.

Also being able to change the index sizes off instances after placing them would also be good, better if it applies to all instances of the same type in the level.