feature/skia merged to master

 Hello,

 as mentioned in the last ESC minutes, I've merged the feature/skia branch to 
master. You ideally should not notice, but in case you do or want to notice, 
here are some details about it:

- Skia is a graphics library, https://skia.org/ .
- The only VCL backends using it by now are the X11 'gen' and the Windows one. 
This in particular means that svp/headless doesn't use it.
- It's disabled by default. Enabling it is similar to OpenGL, so there are 
either GUI options next to the OpenGL ones, or you can do SAL_ENABLESKIA=1.
- A copy of Skia is built by external/skia. You can use --disable-skia to 
completely disable all this.

- It is already more or less working, but it's still work in progress.
- If you use it and find a problem, file a bugreport in bugzilla and assign to 
me.
- Skia works either in raster mode (CPU-based) or uses Vulkan (next-gen 
OpenGL). It needs at least Vulkan 1.1 and a capable driver. Things like 
checking driver capabilities and blacklisting are not implemented yet. In the 
worst case it will just fall back to raster (well, that's not really the 
worst case, it can also cause you screen corruption, X11 freezing or your gfx 
card exploding, but I find especially the last one rather unlikely :) ).
- The LO about dialog includes info on which drawing method is used.

- If this mail is missing some important info, or you want to know more, just 
ask.

-- 
 Luboš Luňák
 l.lunak@collabora.com