Add GUI scale setting and volume contrast setting

GUI scale (#737):
- add display.gui-scale setting (0 - auto, 1..4 - fixed integer scale)
- auto mode picks the largest scale keeping at least 640x480 UI units
- apply scale in GUI (root size, camera, cursor, tooltips, scissors)
- HUD layout and ModelViewer use scaled coordinates
- add GUI Scale trackbar to display settings

Volume contrast (#399):
- add audio.volume-contrast setting (100% - unchanged sounds,
  0% - loud sounds are lowered to the level of quiet ones)
- loudness (RMS) of a sound is calculated once and cached
- add trackbar to audio settings and reset page

src/frontend/hud.hpp

@@ -130,7 +130,7 @@ class Hud : public util::ObjectsKeeper {
     std::shared_ptr<gui::InventoryView> createHotbar();
 
     void processInput(bool visible);
-    void updateElementsPosition(const glm::uvec2& viewport);
+    void updateElementsPosition(const glm::vec2& viewport);
     void updateHotbarControl();
     void cleanup();