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

res/layouts/pages/settings_audio.xml.lua

@@ -1,6 +1,7 @@
-function create_trackbar_setting(id, name, step, postfix)
+function create_trackbar_setting(id, name, step, postfix, tooltip)
     local info = app.get_setting_info(id)
     postfix = postfix or ""
+    tooltip = tooltip or ""
     document.root:add(gui.template("track_setting", {
         id=id,
         name=gui.str(name, "settings"),
@@ -8,7 +9,8 @@ function create_trackbar_setting(id, name, step, postfix)
         min=info.min,
         max=info.max,
         step=step,
-        postfix=postfix
+        postfix=postfix,
+        tooltip=gui.str(tooltip, "settings")
     }))
     update_trackbar_label(app.get_setting(id), id, name, postfix)
 end
@@ -78,6 +80,7 @@ function on_open()
     create_trackbar_setting("audio.volume-ui", "UI Sounds", 0.01)
     create_trackbar_setting("audio.volume-ambient", "Ambient", 0.01)
     create_trackbar_setting("audio.volume-music", "Music", 0.01)
+    create_trackbar_setting("audio.volume-contrast", "Volume Contrast", 0.01, "", "audio.volume-contrast.tooltip")
 
     document.root:add("<label context='settings'>@Microphone</label>")
     document.root:add("<select id='input_device_select' "..