Add handling for demo TR1 levels (#1308)

Demo TR1 levels have the palette after sprite sequences. Attempt to load the level as normal and if it fails load it as the demo version.
Store the original exception and re-throw that if demo fails too.
Closes #1307

trlevel/Level.cpp

@@ -1634,6 +1634,7 @@ namespace trlevel
             }
 
             std::basic_ispanstream<uint8_t> file{ { *bytes } };
+            file.exceptions(std::ios::failbit);
             log_file(activity, file, std::format("Opened file \"{}\"", _filename));
 
             read_header(file, *bytes, activity, callbacks);
@@ -1752,40 +1753,79 @@ namespace trlevel
         _object_textures = read_object_textures_tr1_3(activity, file, callbacks);
         _sprite_textures = read_sprite_textures(activity, file, callbacks);
         _sprite_sequences = read_sprite_sequences(activity, file, callbacks);
-        _cameras = read_cameras(activity, file, callbacks);
-        _sound_sources = read_sound_sources(activity, file, callbacks);
-        const auto boxes = read_boxes_tr1(activity, file, callbacks);
-        read_overlaps(activity, file, callbacks);
-        read_zones_tr1(activity, file, callbacks, static_cast<uint32_t>(boxes.size()));
-        read_animated_textures(activity, file, callbacks);
-        _entities = read_entities_tr1(activity, file, callbacks);
-        read_light_map(activity, file, callbacks);
 
-        read_palette_tr1(file, activity, callbacks);
-        for (const auto& t : _textile8)
+        // Special handling for demo levels or levels where the palette is here:
+        bool on_demo_attempt = false;
+        const auto branch_start = file.tellg();
+        std::optional<std::exception> original_exception;
+
+        do
         {
-            callbacks.on_textile(t.Tile |
-                std::views::transform([&](uint8_t entry_index)
-                    {
-                        // The first entry in the 8 bit palette is the transparent colour, so just return 
-                        // fully transparent instead of replacing it later.
-                        if (entry_index == 0)
-                        {
-                            return 0x00000000u;
-                        }
-                        auto entry = get_palette_entry(entry_index);
-                        uint32_t value = 0xff000000 | entry.Blue << 16 | entry.Green << 8 | entry.Red;
-                        return value;
-                    }) | std::ranges::to<std::vector>());
-        }
-        _textile8 = {};
+            try
+            {
+                if (on_demo_attempt)
+                {
+                    read_palette_tr1(file, activity, callbacks);
+                }
+
+                _cameras = read_cameras(activity, file, callbacks);
+                _sound_sources = read_sound_sources(activity, file, callbacks);
+                const auto boxes = read_boxes_tr1(activity, file, callbacks);
+                read_overlaps(activity, file, callbacks);
+                read_zones_tr1(activity, file, callbacks, static_cast<uint32_t>(boxes.size()));
+                read_animated_textures(activity, file, callbacks);
+                _entities = read_entities_tr1(activity, file, callbacks);
+                read_light_map(activity, file, callbacks);
+
+                if (!on_demo_attempt)
+                {
+                    read_palette_tr1(file, activity, callbacks);
+                }
+
+                for (const auto& t : _textile8)
+                {
+                    callbacks.on_textile(t.Tile |
+                        std::views::transform([&](uint8_t entry_index)
+                            {
+                                // The first entry in the 8 bit palette is the transparent colour, so just return 
+                                // fully transparent instead of replacing it later.
+                                if (entry_index == 0)
+                                {
+                                    return 0x00000000u;
+                                }
+                                auto entry = get_palette_entry(entry_index);
+                                uint32_t value = 0xff000000 | entry.Blue << 16 | entry.Green << 8 | entry.Red;
+                                return value;
+                            }) | std::ranges::to<std::vector>());
+                }
+                _textile8 = {};
+
+                read_cinematic_frames(activity, file, callbacks);
+                read_demo_data(activity, file, callbacks);
+                _sound_map = read_sound_map_tr1(activity, file, callbacks);
+                _sound_details = read_sound_details(activity, file, callbacks);
+                _sound_data = read_sound_data(activity, file, callbacks);
+                _sample_indices = read_sample_indices(activity, file, callbacks);
+                break;
+            }
+            catch (const std::exception& e)
+            {
+                if (!on_demo_attempt)
+                {
+                    original_exception = e;
+                    callbacks.on_progress("Attempting to load as TR1 demo");
+                    file.clear();
+                    file.seekg(branch_start, std::ios::beg);
+                    on_demo_attempt = true;
+                }
+                else
+                {
+                    throw original_exception.value_or(e);
+                }
+            }
+
+        } while (on_demo_attempt);
 
-        read_cinematic_frames(activity, file, callbacks);
-        read_demo_data(activity, file, callbacks);
-        _sound_map = read_sound_map_tr1(activity, file, callbacks);
-        _sound_details = read_sound_details(activity, file, callbacks);
-        _sound_data = read_sound_data(activity, file, callbacks);
-        _sample_indices = read_sample_indices(activity, file, callbacks);
         generate_sounds_tr1(callbacks);
         callbacks.on_progress("Generating meshes");
         generate_meshes(_mesh_data);