Potential crash when starting game with non-default renderer

Renderer is switched to default and back to the selected one during
reset on the emu thread. The render thread may pick up this change.

core/cfg/option.h

@@ -403,6 +403,11 @@ public:
 #endif
 
 	RenderType& operator=(const RenderType& v) { set(v); return value; }
+
+	void reset() override {
+		// don't reset the value to avoid quick switching when starting a game
+		overridden = false;
+	}
 };
 extern RendererOption RendererType;
 extern Option<bool> UseMipmaps;