WPF interop support for Windows/DirectX.

This pull request adds WPF interop support for MonoGame to show graphics in a WPF D3DImage. 

Notable changes:
Added support for Bgr32 and Bgra32 surface formats.
Added support to created a "shared" render target.
Changed the GraphicsDevice constructor. (This is necessary because in WPF projects we want to create GraphicsDevice instances without using the GraphicsDeviceManager. This was not possible because the internal method GraphicsDevice.Initialize() had to be called.)

We will provide a sample which shows how to use MonoGame in WPF.

We have tested the interop in our WPF projects for a while and it seems to work fine.