Content Pipeline Fixes

This is a bunch of content pipeline fixes developed during the process of getting model support functional.  

 - MGCB now supports relative assembly paths.
 - MGCB now dumps the inner exception when it fails.
 - `IndirectPositionCollection` wasn't doing the "indirect" part of its work.
 - Added the `MeshHelper` class with stubs for unimplemented members.
 - Added our implementation of `MeshHelper.CalculateTangentFrames`.
 - Added `MeshHelper.FindSkeleton`.
 - Added `MeshHelper.SwapWindingOrder`.
 - Fixed `VertexContent.VertexCount` to return the correct count.
 - Fixed `VertexBufferContent` to write any vertex format.
 - Added support for `ArrayWriter` to `ContentCompiler`.
 - Fixed `ArrayWriter.GetRuntimeReader` to properly encode to element type.

Note while i have some changes to `FbxImporter` in here that work will be discarded.  @JamesLupiani has a complete replacement FbxImporter that we have been testing and will push up soon.