[.NET] reset 'render-background-color' to transparent

**Background**
We are using the renderer from a .Net application - works great. Currently we like to customize the renderer to let the user change various settings using a dialog. But so far i found no way to set the "render-background-color" back to the default value (transparent).
But the indigo.setOption("render-background-color", ...); accepts (as far as i can see) only RGB values - no chance to provide a transparency info.

**Solution**
Maybe there is already a way to do that - in this case it will be great if you can add this to the documentation. Otherwise having a pseudo value to set the value back to the default.

Again thanks a lot for the great tool.

Start by checking the existing Indigo setOption API and documentation for render-background-color, focusing on how the transparent default is represented. Determine whether the current interface can restore transparency or whether a documented reset value is needed; done means users can reliably return this option to its default state.

Written by the indexing model from the issue text.