[lua] Add new 'beforepaintemptytilemap' app event

Now a tileManagementPlugin can customize the creation of empty tiles
when the user starts drawing on an empty cel.

To implement: https://github.com/aseprite/Attachment-System/issues/119
Related to: https://github.com/aseprite/Attachment-System/issues/127

src/app/app.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2022  Igara Studio S.A.
+// Copyright (C) 2018-2023  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -127,6 +127,12 @@ namespace app {
     obs::signal<void()> ColorSpaceChange;
     obs::signal<void()> PalettePresetsChange;
 
+    // Signal triggered for TileManagementPlugin that want to create a
+    // tile on-the-fly when the active tilemap cel is empty (it's like
+    // a way to customize the "tilemap/tileset", instead of
+    // Manual/Auto/Semi, the plugin can offer a custom behavior).
+    obs::signal<void()> BeforePaintEmptyTilemap;
+
   private:
     class CoreModules;
     class LoadLanguage;

src/app/script/events_class.cpp

@@ -158,6 +158,7 @@ public:
     BgColorChange,
     BeforeCommand,
     AfterCommand,
+    BeforePaintEmptyTilemap,
   };
 
   AppEvents() {
@@ -174,6 +175,8 @@ public:
       return BeforeCommand;
     else if (std::strcmp(eventName, "aftercommand") == 0)
       return AfterCommand;
+    else if (std::strcmp(eventName, "beforepaintemptytilemap") == 0)
+      return BeforePaintEmptyTilemap;
     else
       return Unknown;
   }
@@ -181,7 +184,8 @@ public:
 private:
 
   void onAddFirstListener(EventType eventType) override {
-    auto ctx = App::instance()->context();
+    auto app = App::instance();
+    auto ctx = app->context();
     auto& pref = Preferences::instance();
     switch (eventType) {
       case SiteChange:
@@ -203,6 +207,10 @@ private:
         m_afterCmdConn = ctx->AfterCommandExecution
           .connect(&AppEvents::onAfterCommand, this);
         break;
+      case BeforePaintEmptyTilemap:
+        m_beforePaintConn = app->BeforePaintEmptyTilemap
+          .connect(&AppEvents::onBeforePaintEmptyTilemap, this);
+        break;
     }
   }
 
@@ -223,6 +231,9 @@ private:
       case AfterCommand:
         m_afterCmdConn.disconnect();
         break;
+      case BeforePaintEmptyTilemap:
+        m_beforePaintConn.disconnect();
+        break;
     }
   }
 
@@ -253,6 +264,10 @@ private:
                          { "params", ev.params() } });
   }
 
+  void onBeforePaintEmptyTilemap() {
+    call(BeforePaintEmptyTilemap);
+  }
+
   // ContextObserver impl
   void onActiveSiteChange(const Site& site) override {
     const bool fromUndo = (site.document() &&
@@ -264,6 +279,7 @@ private:
   obs::scoped_connection m_bgConn;
   obs::scoped_connection m_beforeCmdConn;
   obs::scoped_connection m_afterCmdConn;
+  obs::scoped_connection m_beforePaintConn;
 };
 
 class SpriteEvents : public Events

src/app/ui/editor/standby_state.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2022  Igara Studio S.A.
+// Copyright (C) 2018-2023  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -645,7 +645,15 @@ DrawingState* StandbyState::startDrawingState(
   if (editor->layer()->isTilemap() &&
       editor->sprite()->hasTileManagementPlugin() &&
       !editor->layer()->cel(editor->frame())) {
-    return nullptr;
+    // Trigger event so the plugin can create a cel on-the-fly
+    App::instance()->BeforePaintEmptyTilemap();
+
+    if (!editor->layer() ||
+        !editor->layer()->cel(editor->frame())) {
+      return nullptr;
+    }
+    // In other case, it looks like PaintEmptyTilemap event created
+    // the cel...
   }
   // We need to clear and redraw the brush boundaries after the
   // first mouse pressed/point shape if drawn. This is to avoid