video_core: Allow changing background color while emulation is running

As @jroweboy stated, this is just one more atomic in VideoCore.

src/video_core/video_core.cpp

@@ -22,6 +22,7 @@ std::atomic<bool> g_shader_jit_enabled;
 std::atomic<bool> g_hw_shader_enabled;
 std::atomic<bool> g_hw_shader_accurate_gs;
 std::atomic<bool> g_hw_shader_accurate_mul;
+std::atomic<bool> g_renderer_bg_color_update_requested;
 
 /// Initialize the video core
 bool Init(EmuWindow* emu_window) {