airfield gfx improvements

- added separate idle shp to allow using all 8 frames while keeping idle/active separate.
- replaced some empty/background pixels inside tower with shadow.
silo gfx improvements

- fixed some minor errors on the undamaged frames.
- added shadow.
changes civ pistol explosion to single piff

multiple piffs from a civilian pistol shot? Looks a little strange ;)
changes light tank bullet to 50cal.shp

a better match for the low damage per shot, imo.
adjusting explosion position

- originally, frag1 was not centric and appears to the right of the impact/exploding vehicle.
- fball1 appeared quite a bit below the impact point. This moves it up a little.
Yak shp fix

Originally, the 2nd frame of yak.shp used greyscale for the tail fins instead of remap.
gap gen & mgg with shadows
crates with shadow and wood crate palette fix

- westwood never bothered converting the wood crate to RA's palette. I did.
- shadows added to both wood crate and silver crate.
construction yard with shadow

It always annoyed me that the CY had no shadow at all. This adds some at the edges.
adding myself to AUTHORS
Merge pull request #3320 from reaperrr/improved_visuals_v2

several visual improvements