feat: add pig spawn egg item

Add pig spawn egg asset and texture, parse spawn egg item kind and creature property, and spawn the configured entity when used on an empty block. Also fix item texture loading to use actual image dimensions and update selected item UI sizing.

include/Cubed/gameplay/item.hpp

@@ -29,7 +29,7 @@ inline constexpr ItemKind get_item_kind(std::string_view kind) {
     if (kind == "block") {
         return ItemKind::BLOCK;
     }
-    if (kind == "SPAWN_EGG") {
+    if (kind == "spawn_egg") {
         return ItemKind::SPAWN_EGG;
     }
     return ItemKind::NONE;