Share settings page helpers, add GUI cursor/scale API

- move create_trackbar_setting/update_trackbar_label/create_checkbox from
  settings_audio/display/graphics pages to core:settings_common module
- GUI scale trackbar shows the really applied scale, e.g. '3 (effective 2)'
- add gui.get_scale(), gui.get_max_scale(), gui.get_cursor_pos() (UI units);
  document that input.get_mouse_pos() returns window pixels
- InventoryView default color is defined once (InventoryView::DEFAULT_COLOR)
  instead of duplicated literals; 'color' attribute still overrides it

src/graphics/ui/GUI.cpp

@@ -93,13 +93,18 @@ void GUI::onAssetsLoad(Assets* assets) {
 static constexpr glm::uvec2 MIN_UI_SIZE {640, 480};
 static constexpr int MAX_SCALE = 4;
 
-int GUI::calcScale(const glm::uvec2& viewport) const {
+int GUI::calcMaxScale(const glm::uvec2& viewport) {
     int maxFit = 1;
     while (maxFit < MAX_SCALE &&
            viewport.x / (maxFit + 1) >= MIN_UI_SIZE.x &&
            viewport.y / (maxFit + 1) >= MIN_UI_SIZE.y) {
         maxFit++;
     }
+    return maxFit;
+}
+
+int GUI::calcScale(const glm::uvec2& viewport) const {
+    int maxFit = calcMaxScale(viewport);
     int value = engine.getSettings().display.guiScale.get();
     if (value <= 0) {
         return maxFit; // auto
@@ -111,6 +116,10 @@ int GUI::getScale() const {
     return scale;
 }
 
+int GUI::getMaxScale() const {
+    return calcMaxScale(engine.getWindow().getSize());
+}
+
 CursorState GUI::getCursor() const {
     auto cursor = input.getCursor();
     cursor.pos /= static_cast<float>(scale);

src/graphics/ui/GUI.hpp

@@ -95,6 +95,7 @@ namespace gui {
         bool doubleClicked = false;
         bool debug = false;
 
+        static int calcMaxScale(const glm::uvec2& viewport);
         int calcScale(const glm::uvec2& viewport) const;
         CursorState getScaledCursor() const;
 
@@ -134,6 +135,9 @@ namespace gui {
         /// @brief Get current GUI scale: window pixels per one UI unit
         int getScale() const;
 
+        /// @brief Get maximal GUI scale that fits the current window size
+        int getMaxScale() const;
+
         /// @brief Get cursor state in UI coordinates
         CursorState getCursor() const;
 

src/graphics/ui/elements/InventoryView.cpp

@@ -78,7 +78,7 @@ void InventoryBuilder::addGrid(
     if (addpanel) {
         auto panel =
             std::make_shared<gui::Container>(gui, glm::vec2(width, height));
-        view->setColor(glm::vec4(0.122f, 0.122f, 0.122f, 0.878f));
+        view->setColor(InventoryView::DEFAULT_COLOR);
         view->add(panel, pos);
     }
 

src/graphics/ui/elements/InventoryView.hpp

@@ -160,6 +160,8 @@ namespace gui {
 
         static const int SLOT_INTERVAL = 4;
         static const int SLOT_SIZE = ITEM_ICON_SIZE;
+        /// @brief Default background color (can be overridden by 'color' attribute)
+        static inline const glm::vec4 DEFAULT_COLOR {0.122f, 0.122f, 0.122f, 0.878f};
     };
 
     class InventoryBuilder {

src/graphics/ui/gui_xml.cpp

@@ -860,7 +860,7 @@ static std::shared_ptr<UINode> read_inventory(
     UiXmlReader& reader, const xml::xmlelement& element
 ) {
     auto view = std::make_shared<InventoryView>(reader.getGUI());
-    view->setColor(glm::vec4(0.122f, 0.122f, 0.122f, 0.878f));  // TODO: fixme
+    view->setColor(InventoryView::DEFAULT_COLOR);
     reader.addIgnore("slot");
     reader.addIgnore("slots-grid");
     reader.readUINode(reader, element, *view);

src/logic/scripting/lua/libs/libgui.cpp

@@ -1067,6 +1067,20 @@ static int l_gui_getviewport(lua::State* L) {
     return lua::pushvec2(L, engine->getGUI().getContainer()->getSize());
 }
 
+static int l_gui_get_scale(lua::State* L) {
+    return lua::pushinteger(L, engine->getGUI().getScale());
+}
+
+static int l_gui_get_max_scale(lua::State* L) {
+    return lua::pushinteger(L, engine->getGUI().getMaxScale());
+}
+
+static int l_gui_get_cursor_pos(lua::State* L) {
+    if (engine->isHeadless())
+        return 0;
+    return lua::pushvec2(L, engine->getGUI().getCursor().pos);
+}
+
 static int l_gui_clear_markup(lua::State* L) {
     auto lang = lua::require_string(L, 1);
     std::string text = lua::require_string(L, 2);
@@ -1247,6 +1261,9 @@ static int l_screenshot(lua::State* L) {
 const luaL_Reg guilib[] = {
     {"screenshot", lua::wrap<l_screenshot>},
     {"get_viewport", lua::wrap<l_gui_getviewport>},
+    {"get_scale", lua::wrap<l_gui_get_scale>},
+    {"get_max_scale", lua::wrap<l_gui_get_max_scale>},
+    {"get_cursor_pos", lua::wrap<l_gui_get_cursor_pos>},
     {"getattr", lua::wrap<l_gui_getattr>},
     {"setattr", lua::wrap<l_gui_setattr>},
     {"get_env", lua::wrap<l_gui_get_env>},