renderer: Add Vulkan renderer to the list of renderers

It's better than the Cairo renderer, so use it instead.

It's still only picked once GL fails, so it will probably only ever be
picked when people use GDK_DEBUG=gl-disable, but at least it will be
picked.

gsk/gskrenderer.c

@@ -582,6 +582,16 @@ get_renderer_for_gl (GdkSurface *surface)
   return GSK_TYPE_GL_RENDERER;
 }
 
+static GType
+get_renderer_for_vulkan (GdkSurface *surface)
+{
+#ifdef GDK_RENDERING_VULKAN
+  return GSK_TYPE_VULKAN_RENDERER;
+#else
+  return G_TYPE_INVALID;
+#endif
+}
+
 static GType
 get_renderer_fallback (GdkSurface *surface)
 {
@@ -595,6 +605,7 @@ static struct {
   { get_renderer_for_env_var },
   { get_renderer_for_backend },
   { get_renderer_for_gl },
+  { get_renderer_for_vulkan },
   { get_renderer_fallback },
 };