add 'fallback' property to xml and scripting

res/layouts/templates/pack.xml

@@ -11,5 +11,5 @@
     <label ifnot='%{error}' pos='80,28' size='355,50' multiline='true' color='#FFFFFFB2'>
         %{description}
     </label>
-    <image pos='8,8' src='%{icon}' size='64'/>
+    <image pos='8,8' src='%{icon}' fallback='gui/no_icon' size='64'/>
 </container>

res/layouts/templates/world.xml

@@ -4,7 +4,7 @@
     hover-color='#162B3399'
     onclick='app.open_world("%{name}")'
 >
-    <image pos='1' src='%{icon}' size='96,64'></image>
+    <image pos='1' src='%{icon}' fallback='gui/no_world_icon' size='96,64'></image>
     <label pos='104,8'>%{name}</label>
     <button margin='4'
             gravity='center-right'

src/graphics/ui/elements/Image.hpp

@@ -14,7 +14,6 @@ namespace gui {
         std::string fallback;
         UVRegion region {};
         bool autoresize = false;
-        bool isFallback = false;
 
         util::TextureRegion refreshTexture(const Assets& assets);
     public:

src/graphics/ui/gui_xml.cpp

@@ -632,8 +632,10 @@ static std::shared_ptr<UINode> read_image(
     const UiXmlReader& reader, const xml::xmlelement& element
 ) {
     std::string src = element.attr("src", "").getText();
-    auto image = std::make_shared<Image>(reader.getGUI(), src);
+    std::string fallback = element.attr("fallback", "").getText();
+    auto image = std::make_shared<Image>(reader.getGUI(), std::move(src));
     read_uinode(reader, element, *image);
+    image->setFallback(std::move(fallback));
 
     if (element.has("region")) {
         auto vec = element.attr("region").asVec4();

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

@@ -397,6 +397,13 @@ static int p_get_src(UINode* node, lua::State* L) {
     return 0;
 }
 
+static int p_get_fallback(UINode* node, lua::State* L) {
+    if (auto image = dynamic_cast<Image*>(node)) {
+        return lua::pushstring(L, image->getFallback());
+    }
+    return 0;
+}
+
 static int p_get_region(UINode* node, lua::State* L) {
     if (auto image = dynamic_cast<Image*>(node)) {
         const auto& region = image->getRegion();
@@ -663,6 +670,7 @@ static int l_gui_getattr(lua::State* L) {
             {"syntax", p_get_syntax},
             {"markup", p_get_markup},
             {"src", p_get_src},
+            {"fallback", p_get_fallback},
             {"value", p_get_value},
             {"min", p_get_min},
             {"max", p_get_max},
@@ -790,6 +798,11 @@ static void p_set_src(UINode* node, lua::State* L, int idx) {
         modelviewer->setModel(lua::require_string(L, idx));
     }
 }
+static void p_set_fallback(UINode* node, lua::State* L, int idx) {
+    if (auto image = dynamic_cast<Image*>(node)) {
+        image->setFallback(lua::require_string(L, idx));
+    }
+}
 static void p_set_region(UINode* node, lua::State* L, int idx) {
     if (auto image = dynamic_cast<Image*>(node)) {
         auto vec = lua::tovec4(L, idx);
@@ -962,6 +975,7 @@ static int l_gui_setattr(lua::State* L) {
             {"syntax", p_set_syntax},
             {"markup", p_set_markup},
             {"src", p_set_src},
+            {"fallback", p_set_fallback},
             {"caret", p_set_caret},
             {"value", p_set_value},
             {"min", p_set_min},