Minor preparations for being able to dynamically update UI scale

A lot of things in the game does not adapt to this quite yet

src/main.cc

@@ -1280,8 +1280,7 @@ tproject_init(void)
     setup_opengl_settings();
     settings.save();
 
-    _tms.xppcm *= settings["uiscale"]->v.f;
-    _tms.yppcm *= settings["uiscale"]->v.f;
+    P.update_uiscale(settings["uiscale"]->v.f);
 
 #ifdef NO_UI
     settings["render_gui"]->set(false);
@@ -1989,6 +1988,26 @@ principia::add_action(int id, void *data)
     SDL_UnlockMutex(P.action_mutex);
 }
 
+static bool has_updated_uiscale = false;
+static float base_xppcm = 0.f;
+static float base_yppcm = 0.f;
+
+void principia::update_uiscale(float scale) {
+    if (!has_updated_uiscale) {
+        base_xppcm = _tms.xppcm;
+        base_yppcm = _tms.yppcm;
+    }
+
+    _tms.xppcm = base_xppcm * scale;
+    _tms.yppcm = base_yppcm * scale;
+
+    if (!has_updated_uiscale) {
+        has_updated_uiscale = true;
+    } else {
+        tproject_window_size_changed();
+    }
+}
+
 tvec3
 principia::get_light_normal()
 {

src/main.hh

@@ -195,6 +195,8 @@ extern class principia
     };
 
     tvec3 get_light_normal();
+
+    void update_uiscale(float new_uiscale);
 } P;
 
 struct principia_action

src/ui_gtk3.cc

@@ -4955,6 +4955,11 @@ save_settings()
         save_setting_row(r);
     }
 
+#if 0
+    // TODO: Dynamic update of UI scale without restart
+    P.update_uiscale(settings["uiscale"]->v.f);
+#endif
+
     sm::load_settings();
 
     strcpy(tmp, get_cb_val(settings_shadow_res));