gsk: Change the default renderer
The intent of this change to get wider testing and verify that the new renderers are production-ready. If significant problems show up, we will revert this change for 4.14.
The new preference order is ngl > gl > vulkan > cairo.
The gl renderer is still there because we need it to support gles2 systems, and vulkan still has some rough edges in application support (no gl area support, webkit only works with gl).
If you need to override the default renderer choice, you can still use the GSK_RENDERER environment variable.