add ui and viewport scale option, fix combobox items height from parent

src/app.h

@@ -139,13 +139,8 @@ public:
     glm::vec2 cursor{ 0, 0 };
     glm::vec2 gesture_p0;
     glm::vec2 gesture_p1;
-#ifdef __ANDROID__
-    float zoom = 3.0;
-#elif __IOS__
-    float zoom = 2.0;
-#else
-    float zoom = 1.0;
-#endif // __ANDROID__
+    float display_density = 1.f;
+    float zoom = 1.f;
     
 #if defined(__IOS__) && defined(__OBJC__)
     GameViewController* ios_view;