PVRTC support added to pipeline.

A couple of questions came up while integrating this:

Ended up making a PVRTCBitmapContent, since it will need different implementations of TryCopyTo/From(). Seeing all of the texture formats we'll end up supporting, should we have a single CompressedBitmapContent class that implements the different texture formats?

How much control are we giving to the user when it comes to texture formats. Technically, PVRTC and DXT can work on Android. What would be the right way of choosing the output format?

PVRTC can do either 2BPP or 4BPP. Where should this be exposed to the user? For now I just forced 4BPP.

Changes in this PR allow us to move PVRTexLibC out of MonoGame's repo, and into it's own. This way we open the possibility of other developers working on it. I'll be submitting a PR to the Dependency repo that moves these .dll's from Tools to ThirdParty. Don't merge this until that's complete.