Dropping X11 VCL backend

[Ilmari Lauhakangas]

On 29.12.2020 8.13, Chris Sherlock wrote:
> I am interested in knowing the future of the X11 VCL backend now that we have a Skia backend. Are there any plans afoot to remove it and just go with the Skia backend?
> 
> If not, may I ask the reason? It seems maintenance heavy.
> 
> If we are, do we have a potential release where we believe that Skia is stable enough to use and remove the X11 backend?

We could start by gathering the concrete things that we use X11 backend for.

One of them is UI tests, where we advise to use SAL_USE_VCLPLUGIN=gen 
when we want to observe the UI while running tests or while logging UI 
events in the domain specific language: 
https://wiki.documentfoundation.org/Development/UITests

Hopefully someone can explain why we still need to do this.

Ilmari