Introduces minibibs + code logic to ground bibless "hovering" buildings

See discussion @4442.

This adds little dirt-patches to ground bib-less hovering buildings:
Chronosphere, Iron Curtain, Missile Silo, Ore Silo, Airfield, Service Depot, and many defense buildings.
Each tileset (desert, interior, snow, temperat) has it's own special bibs; made sure interior looks good on both it's ground types, the metal and the wooden one.

Was a LOT of work, but well worth it tho, as IMO it simply looks great! I'm very pleased with the results, see screenies below.
It's also more realistic. I tend do imagine snow or grass around a building is not totally "untouched" when operating people get in and out, especially in a military base.

The minibib does not take an additional cell below the building like the normal bib, it only acts as background arround the sprite.

I aimed for the best compromise between visual looks and fitting em to the cell-structure, so the player always gets a feel for the cell squares, even with buildings that are rotated by several degrees.
So it also serves gameplay.

Will ad more randomly selectable bibs in the future for the 1x1 bibs, so each building's dirt pile is unique!

Code-wise, I decided against adding an unnecessary additional "minibib"-trait, but to use the already existing code for the bib with only minimal changes (to check if it has the "minibib"-sequence in the yaml-file). I think this is the cleanest approach.

Changed some offset-values in sequences\structures.yaml to make some minibibs look better.
Also fixed the service depot-footprint and offset it a bit, so it looks good and finally has its xxx, xxx footprint which you would expect.

Minibib does not look good on walls by principle, on sam (needs a special bib which i will commit some time in the future) and camo pillbox (interferes with camo transparency), so these buildings are an exeption.

Also, since it is looking so much better, i changed Radar dome & tech center's bib to minibibs.
You almost always have only one of these buildings, so this will not affect gameplay in any way.

Screenshots:
![temperat](https://f.cloud.github.com/assets/1968396/2148252/645bb648-93e1-11e3-85a6-b5f45ac3b62f.PNG)
![snow](https://f.cloud.github.com/assets/1968396/2148257/77fcd984-93e1-11e3-86e7-62a45886b71b.PNG)
![desert](https://f.cloud.github.com/assets/1968396/2148260/8414821c-93e1-11e3-957b-ff5de74a7acd.PNG)
![interior](https://f.cloud.github.com/assets/1968396/2148265/98b6d486-93e1-11e3-9df0-0741195beae7.PNG)