LibreOffice run modes (headlessness)

[Jan-Marek Glogowski]

After a long discussion on IRC, I'm replying to myself to add you some update.

Am 09.11.18 um 14:16 schrieb Jan-Marek Glogowski:
> --run-mode=visible|hidden|headless(|console)
> 
> 1. visible: normal GUI (default)
> 2. hidden: like normal, but start hidden / no window
> 3. headless: can not become visible; still uses OS GUI functions, so needs GUI
> (4. console: can not become visible, as it doesn't use GUI functions at all)
> 
> * AKA level of visibility / headlessness
> * Level 4 is done by SAL_USE_VCLPLUGIN=svp as a virtual GUI, which just uses images for output.
> * Or use console only internally for stuff like gengal?

There will be an additional handling of the SAL_USE_VCLPLUGIN=svp on platforms, which don't have the
extra svp plugin compiled in (currently Win and OSX).

My target is to get a "virtual svp mode" working as an internal mode for the win and osx plugins, as
a better replacement for VCL_HIDE_WINDOWS. Then the tests would completely render to bitmaps, never
exposing any GUI functions and should work via SSH. I'm not sure if we want something like a
"--use-software-renderer" switch in addition, but SAL_USE_VCLPLUGIN=svp is kept for compatibility.

> VCL_HIDE_WINDOWS => drop

The variable is dropped, but replaced by SAL_USE_VCLPLUGIN=svp to be used consistent on all
platforms, which can support it.

Jan-Marek