gsk: Change the default renderer

The intent of this change to get wider testing and verify that the
Vulkan drivers we get to use in the wild are good enough for our
needs. If significant problems show up, we will revert this change
for 4.16.

The new preference order is vulkan > ngl > gl > cairo.

The gl renderer is still there because we need it to support gles2.

If you need to override the default renderer choice, you can
still use the GSK_RENDERER environment variable.

Fixes: #6537

gsk/gskrenderer.c

@@ -703,9 +703,9 @@ static struct {
   { get_renderer_for_display },
   { get_renderer_for_env_var },
   { get_renderer_for_backend },
+  { get_renderer_for_vulkan },
   { get_renderer_for_gl },
   { get_renderer_for_gles2 },
-  { get_renderer_for_vulkan },
   { get_renderer_fallback },
 };