Warn only once per missing icon

Mods may dynamically request icons
(e.g., [icon=\"emblem_${civ}\"], [icon=\"icon_${res}\"]) that
don't exist when another mod introduced the new civ or resource
but hasn't added that icon.
This results in excessive error log spam during normal gameplay.

source/gui/CGUI.cpp

@@ -1333,6 +1333,12 @@ void CGUI::Xeromyces_ReadIcon(const XMBData& xmb, XMBElement element)
 	m_Icons.emplace(name, std::move(icon));
 }
 
+void CGUI::ReportMissingIcon(const CStr& name) const
+{
+	if (m_MissingIconsReported.insert(name).second)
+		LOGWARNING("Trying to use an icon, imgleft or imgright-tag with an undefined icon (\"%s\").", name);
+}
+
 void CGUI::Xeromyces_ReadTooltip(const XMBData& xmb, XMBElement element)
 {
 	std::unique_ptr<IGUIObject> object{std::make_unique<CTooltip>(*this)};

source/gui/CGUI.h

@@ -264,6 +264,15 @@ public:
 	 */
 	const SGUIIcon& GetIcon(const CStr& name) const { return m_Icons.at(name); }
 
+	/**
+	 * Log a warning when text markup refers to an icon that doesn't exist.
+	 * Mods may reference icons dynamically (e.g. an icon name
+	 * built from a civ or resource identifier) that can be temporarily
+	 * missing while dependent mods are updated together; each distinct
+	 * missing icon name is only logged once per GUI page to avoid log spam.
+	 */
+	void ReportMissingIcon(const CStr& name) const;
+
 	/**
 	 * Check if a style exists
 	 */
@@ -725,6 +734,10 @@ private:
 	// Icons
 	std::map<CStr, const SGUIIcon> m_Icons;
 
+	// Icons that have already been reported as missing, to avoid repeating
+	// the same warning every time referencing text is regenerated.
+	mutable std::unordered_set<CStr> m_MissingIconsReported;
+
 public:
 	struct ModuleArtifact
 	{

source/gui/SettingTypes/CGUIString.cpp

@@ -113,7 +113,7 @@ void CGUIString::GenerateTextCall(const CGUI& pGUI, SFeedback& Feedback, CStrInt
 			if (!pGUI.HasIcon(path))
 			{
 				if (pObject)
-					LOGERROR("Trying to use an icon, imgleft or imgright-tag with an undefined icon (\"%s\").", path.c_str());
+					pGUI.ReportMissingIcon(path);
 				continue;
 			}