Added FreeImage binaries

This PR contains both the binary and C# wrapper for the FreeImage library, to be implemented in MGCB.

The 64-bit version requires the source to be downloaded and compiled from Visual Stuio. These binaries are built from r3160. The C# Wrapper library was built against AnyCPU.

No changes or hacks were needed to get this working.

This is part of https://github.com/mono/MonoGame/issues/2478. When this is merged, I'll submit another PR that adds FreeImage support into our TextureImporter.