Swap to using the Vulkan renderer by default

This will break a lot of games and fix others, but it makes more sense to use a backend being actively worked on.

src/xenia/app/xenia_main.cc

@@ -80,7 +80,7 @@ std::unique_ptr<gpu::GraphicsSystem> CreateGraphicsSystem() {
     std::unique_ptr<gpu::GraphicsSystem> best;
 
     best = std::unique_ptr<gpu::GraphicsSystem>(
-        new xe::gpu::gl4::GL4GraphicsSystem());
+        new xe::gpu::vulkan::VulkanGraphicsSystem());
     if (best) {
       return best;
     }