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/graphics/core/DrawContext.hpp

@@ -24,6 +24,7 @@ class DrawContext {
     BlendMode blendMode = BlendMode::normal;
     int scissorsCount = 0;
     float lineWidth = 1.0f;
+    float uiScale = 1.0f;
     std::array<const Bindable*, advanced_pipeline::TARGETS_COUNT> textures {};
 public:
     DrawContext(
@@ -46,5 +47,9 @@ public:
     void setBlendMode(BlendMode mode);
     void setScissors(const glm::vec4& area);
     void setLineWidth(float width);
+
+    /// @brief Set scale of UI coordinates (UI units -> window pixels).
+    /// Affects setScissors only
+    void setUiScale(float scale);
     void useTexture(int target, const Bindable* texture);
 };