Clean up some rendering-related resources in DeInit (#1836)

- The unique_ptr destructors will take care of the cleanup

src/GPU.cpp

@@ -176,6 +176,8 @@ void DeInit()
     Framebuffer[0][1] = nullptr;
     Framebuffer[1][0] = nullptr;
     Framebuffer[1][1] = nullptr;
+
+    CurGLCompositor = nullptr;
 }
 
 void ResetVRAMCache()