video_core: gpu: Refactor out synchronous/asynchronous GPU implementations.

- We must always use a GPU thread now, even with synchronous GPU.

src/video_core/gpu_thread.h

@@ -10,8 +10,9 @@
 #include <optional>
 #include <thread>
 #include <variant>
+
 #include "common/threadsafe_queue.h"
-#include "video_core/gpu.h"
+#include "video_core/framebuffer_config.h"
 
 namespace Tegra {
 struct FramebufferConfig;
@@ -25,6 +26,10 @@ class GraphicsContext;
 class System;
 } // namespace Core
 
+namespace VideoCore {
+    class RendererBase;
+} // namespace VideoCore
+
 namespace VideoCommon::GPUThread {
 
 /// Command to signal to the GPU thread that processing has ended