Resolve memory leak when deleting a webgl layer

Various references were kept, preventing the layer and underlying
renderer and webgl context to be garbage collected.

Also, the Helper was simplified because it turns out deleting manually
all Webgl objects is useless: these objects will be released when
the context is garbage collected anyway.

Note: this touches the Layer and BaseLayer classes, as the following were
preventing the layer from being garbage collected:
* layer reference in the `state_` object in BaseLayer
* dangling listener for source change in Layer

src/ol/webgl/Helper.js

@@ -266,18 +266,6 @@ class WebGLHelper extends Disposable {
      */
     this.bufferCache_ = {};
 
-    /**
-     * @private
-     * @type {!Array<WebGLShader>}
-     */
-    this.shaderCache_ = [];
-
-    /**
-     * @private
-     * @type {!Array<WebGLProgram>}
-     */
-    this.programCache_ = [];
-
     /**
      * @private
      * @type {WebGLProgram}
@@ -419,18 +407,6 @@ class WebGLHelper extends Disposable {
   disposeInternal() {
     this.canvas_.removeEventListener(ContextEventType.LOST, this.boundHandleWebGLContextLost_);
     this.canvas_.removeEventListener(ContextEventType.RESTORED, this.boundHandleWebGLContextRestored_);
-    const gl = this.getGL();
-    if (!gl.isContextLost()) {
-      for (const key in this.bufferCache_) {
-        gl.deleteBuffer(this.bufferCache_[key].buffer);
-      }
-      for (const key in this.programCache_) {
-        gl.deleteProgram(this.programCache_[key]);
-      }
-      for (const key in this.shaderCache_) {
-        gl.deleteShader(this.shaderCache_[key]);
-      }
-    }
   }
 
   /**
@@ -652,7 +628,6 @@ class WebGLHelper extends Disposable {
     const shader = gl.createShader(type);
     gl.shaderSource(shader, source);
     gl.compileShader(shader);
-    this.shaderCache_.push(shader);
     return shader;
   }
 
@@ -684,7 +659,6 @@ class WebGLHelper extends Disposable {
     gl.attachShader(program, fragmentShader);
     gl.attachShader(program, vertexShader);
     gl.linkProgram(program);
-    this.programCache_.push(program);
     return program;
   }
 
@@ -811,8 +785,6 @@ class WebGLHelper extends Disposable {
    */
   handleWebGLContextLost() {
     clear(this.bufferCache_);
-    clear(this.shaderCache_);
-    clear(this.programCache_);
     this.currentProgram_ = null;
   }
 
@@ -823,8 +795,6 @@ class WebGLHelper extends Disposable {
   handleWebGLContextRestored() {
   }
 
-  // TODO: shutdown program
-
   /**
    * Will create or reuse a given webgl texture and apply the given size. If no image data
    * specified, the texture will be empty, otherwise image data will be used and the `size`