gpu: Refactor a/synchronous implementations into their own classes.

src/video_core/CMakeLists.txt

@@ -17,6 +17,10 @@ add_library(video_core STATIC
     engines/shader_header.h
     gpu.cpp
     gpu.h
+    gpu_asynch.cpp
+    gpu_asynch.h
+    gpu_synch.cpp
+    gpu_synch.h
     gpu_thread.cpp
     gpu_thread.h
     macro_interpreter.cpp