Update WPF structs to compute more inclusive hashes

In CoreCLR, the GetHashCode logic for structs with float/double parameters has changed and now only uses the first field for hash computation (see [this](https://github.com/dotnet/corefx/issues/35613) for more details).
As [recommended](https://github.com/dotnet/corefx/issues/35613#issuecomment-467726210), we should go through these structs to override and implement our own hashes. These new hashes should be more inclusive in order to better mirror what the behaviour was like in .NET Framework. There is already an example of this being done [here](https://github.com/dotnet/wpf/pull/1308) for the MILColorF struct.