Adding support for TextureFormatOptions to FontDescriptionProcessor.

Updated Pipeline so that changes to this value will trigger a SaveEnabled.
This may require a `[DefaultValue]` attribute.  I know XNA required this to display the correct default value in the property grid when the default value was not zero or the equivalent.  I'm not sure if the WinForms PropertyGrid uses this attribute or not. See https://github.com/mono/MonoGame/blob/develop/MonoGame.Framework.Content.Pipeline/Processors/ModelProcessor.cs#L86 for an example.