Skia painter is visibly faster than LibGfx painter and has more complete
CSS transforms support. With this change:
- On Linux, it will try to use Vulkan-backend with fallback to
CPU-backend
- On macOS it will try to use Metal-backend with fallback to
CPU-backend
- headless-browser always runs with CPU-backend in layout mode
These test work with LibGfx painter but won't longer work after
switching to Skia, because it produces slightly different antialiasing,
rounding in color blending, etc.
