WebGL 2: add readPixels API to read pixels into pixel pack buffer

Source/modules/webgl/WebGL2RenderingContextBase.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 typedef unsigned long  GLenum;
 typedef boolean        GLboolean;
 typedef unsigned long  GLbitfield;
 typedef byte           GLbyte; // 'byte' should be a signed 8 bit type.
 typedef short          GLshort;
 typedef long           GLint;
@@ skipping 414 matching lines @@
     GLuint getUniformBlockIndex(WebGLProgram? program, DOMString uniformBlockName);
     [CallWith=ScriptState] any getActiveUniformBlockParameter(WebGLProgram? program, GLuint uniformBlockIndex, GLenum pname);
     DOMString getActiveUniformBlockName(WebGLProgram? program, GLuint uniformBlockIndex);
     void uniformBlockBinding(WebGLProgram? program, GLuint uniformBlockIndex, GLuint uniformBlockBinding);
 
     /* Vertex Array Objects */
     WebGLVertexArrayObject createVertexArray();
     void deleteVertexArray(WebGLVertexArrayObject? vertexArray);
     GLboolean isVertexArray(WebGLVertexArrayObject? vertexArray);
     void bindVertexArray(WebGLVertexArrayObject? vertexArray);
+
+    /* Reading */
+    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);
 };
 WebGL2RenderingContextBase implements WebGLRenderingContextBase;