Changed paradrop shadow

A first attempt at addressing #3335

Have you guys discussed this issue further? How do you feel about the idea of extracting the shadow bits from a sprite into a separate sprite and deleting them from the original sprite? Thus rendering units from two sprites?

Or adding a way to render sprites without shadows and only shadows without the other content?

I've tried to figure out a way to do it using palette manipulation, as suggested by @chrisforbes in #3335, but reading the sprite rendering code proved to be too difficult for me. Maybe if you guys could tell me where and how exactly the magic happens with palette application to the rendered image?

![untitled](https://cloud.githubusercontent.com/assets/5747153/2874690/5035a4b6-d3f7-11e3-9d17-ae16046e82c5.png)

A check for `self.CenterPosition.Z > 0` is more general, and removes the unnecessary coupling between traits.  There's really no need for this manipulation of `ParachuteAttachment.Activated`.