support for alpha cursors

[CendioOssman]

It would be nice to support proper alpha channel for cursors. There is an extension for it:

https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#cursor-with-alpha-pseudo-encoding

And it's implemented by UltraVNC and TigerVNC.

The difficult part is that is uses the normal decoders for the data, so we probably need to restructure things a bit as they currently always send their data to the display object.

[CendioOssman]

There is now also documentation for VMware's cursor extension which also supports this, and with a lot let complexity. I'm working on implementing it in TigerVNC as well.

[CendioOssman]

Support for VMware's variant was added in #1299. That means this feature now works with VMware, and will work with the next version of TigerVNC.

We might want to implement RealVNC's variant as well before closing this issue though. That way it will work with a large number of VNC servers.

[Neustradamus]

To follow this ticket :)