You can now set the boolean preserveDrawingBuffer in the Game Config (either directly, or in the Render Config). This is passed to the WebGL context during creation and controls if the buffers are automatically cleared each frame or not. The default is to clear them. Set to true to retain them.

Author: photonstorm

src/core/Config.js

@@ -393,6 +393,11 @@ var Config = new Class({
          */
         this.clearBeforeRender = GetValue(renderConfig, 'clearBeforeRender', true);
 
+        /**
+         * @const {boolean} Phaser.Core.Config#preserveDrawingBuffer - If the value is true the WebGL buffers will not be cleared and will preserve their values until cleared or overwritten by the author.
+         */
+        this.preserveDrawingBuffer = GetValue(renderConfig, 'preserveDrawingBuffer', false);
+
         /**
          * @const {boolean} Phaser.Core.Config#premultipliedAlpha - In WebGL mode, sets the drawing buffer to contain colors with pre-multiplied alpha.
          */

src/core/typedefs/GameConfig.js

@@ -66,7 +66,8 @@ var WebGLRenderer = new Class({
             premultipliedAlpha: gameConfig.premultipliedAlpha,
             stencil: true,
             failIfMajorPerformanceCaveat: gameConfig.failIfMajorPerformanceCaveat,
-            powerPreference: gameConfig.powerPreference
+            powerPreference: gameConfig.powerPreference,
+            preserveDrawingBuffer: gameConfig.preserveDrawingBuffer
         };
 
         /**