Implement map-specific postcard textures

Celeste.Mod.mm/Mod/Meta/MapMeta.cs

@@ -54,6 +54,7 @@ public MapMeta(BinaryPacker.Element meta) {
         public string CassetteSong { get; set; }
 
         public string PostcardSoundID { get; set; }
+        public string PostcardTexture { get; set; }
 
         public string ForegroundTiles { get; set; }
         public string BackgroundTiles { get; set; }
@@ -244,6 +245,9 @@ public void ApplyTo(patch_AreaData area) {
 
                 if (CassetteModifier != null)
                     meta.CassetteModifier = CassetteModifier;
+
+                if (PostcardTexture != null)
+                    meta.PostcardTexture = PostcardTexture;
             }
         }
 

Celeste.Mod.mm/Patches/LevelEnter.cs

@@ -140,10 +140,17 @@ private IEnumerator ErrorRoutine(string message) {
             Engine.Scene = new OverworldLoader(Overworld.StartMode.AreaQuit);
         }
 
-        private IEnumerator EnterWithPostcardRoutine(string message, string soundId) {
+        private IEnumerator EnterWithPostcardRoutine(string message, string soundId)
+            => EnterWithPostcardRoutine(message, soundId, patch_AreaData.Get(session)?.Meta?.PostcardTexture);
+
+        private IEnumerator EnterWithPostcardRoutine(string message, string soundId, string postcardTexture) {
             yield return 1f;
 
-            Add(postcard = new patch_Postcard(message, soundId));
+            patch_Postcard localPostcard = new patch_Postcard(message, soundId);
+            if (postcardTexture is not null)
+                localPostcard.Postcard = GFX.Gui[postcardTexture];
+
+            Add(postcard = localPostcard);
             yield return postcard.DisplayRoutine();
 
             IEnumerator inner = orig_Routine();

Celeste.Mod.mm/Patches/Postcard.cs

@@ -1,7 +1,18 @@
-using MonoMod;
+using Monocle;
+using MonoMod;
 
 namespace Celeste {
     class patch_Postcard : Postcard {
+        // make the vanilla field accessible to our patch class
+        private MTexture postcard;
+
+        /// <summary>
+        ///   The texture of the postcard.
+        /// </summary>
+        public MTexture Postcard {
+            get => postcard;
+            set => postcard = value;
+        }
 
         public patch_Postcard(string msg, int area)
             : base(msg, area) {
@@ -50,6 +61,5 @@ public void ctor(string msg, string soundId) {
 
             ctor(msg, prefix + "_in", prefix + "_out");
         }
-
     }
 }