Add clearCanvas flag to RuntimeScene for multi-scene rendering support (#5525)

* Add `clearCanvas` field to `RuntimeScene` class and pass it to `PIXI.Renderer.render` as `clear` option to support multi-scene rendering.

* Show in developer changelog

GDJS/Runtime/pixi-renderers/runtimescene-pixi-renderer.ts

@@ -119,7 +119,7 @@ namespace gdjs {
               // Render the background color.
               pixiRenderer.backgroundColor = this._runtimeScene.getBackgroundColor();
               pixiRenderer.backgroundAlpha = 1;
-              pixiRenderer.clear();
+              if (this._runtimeScene.getClearCanvas()) pixiRenderer.clear();
 
               isFirstRender = false;
             }
@@ -192,7 +192,7 @@ namespace gdjs {
                   this._runtimeScene.getBackgroundColor()
                 );
                 threeRenderer.resetState();
-                threeRenderer.clear();
+                if (this._runtimeScene.getClearCanvas()) threeRenderer.clear();
                 threeScene.background = new THREE.Color(
                   this._runtimeScene.getBackgroundColor()
                 );
@@ -251,7 +251,9 @@ namespace gdjs {
         // Render all the layers then.
         // TODO: replace by a loop like in 3D?
         pixiRenderer.backgroundColor = this._runtimeScene.getBackgroundColor();
-        pixiRenderer.render(this._pixiContainer);
+        pixiRenderer.render(this._pixiContainer, {
+          clear: this._runtimeScene.getClearCanvas(),
+        });
         this._layerRenderingMetrics.rendered2DLayersCount++;
       }
 

GDJS/Runtime/runtimescene.ts

@@ -32,6 +32,10 @@ namespace gdjs {
     _requestedChange: SceneChangeRequest;
     /** Black background by default. */
     _backgroundColor: integer = 0;
+
+    /** Should the canvas be cleared before this scene rendering. */
+    _clearCanvas: boolean = true;
+
     _onceTriggers: OnceTriggers;
     _profiler: gdjs.Profiler | null = null;
 
@@ -524,6 +528,22 @@ namespace gdjs {
       return this._backgroundColor;
     }
 
+    /**
+     * Set whether the canvas should be cleared before this scene rendering.
+     * This is experimental: if possible, try to avoid relying on this and use
+     * custom objects to build complex scenes.
+     */
+    setClearCanvas(shouldClearCanvas: boolean): void {
+      this._clearCanvas = shouldClearCanvas;
+    }
+
+    /**
+     * Get whether the canvas should be cleared before this scene rendering.
+     */
+    getClearCanvas(): boolean {
+      return this._clearCanvas;
+    }
+
     /**
      * Get the name of the scene.
      */