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

Source/modules/webgl/WebGL2RenderingContextBase.idl

Index: Source/modules/webgl/WebGL2RenderingContextBase.idl
diff --git a/Source/modules/webgl/WebGL2RenderingContextBase.idl b/Source/modules/webgl/WebGL2RenderingContextBase.idl
index 3361c1aa65bdefc8b14b2001053cad60ca5f1c0d..f8a77afaa084e17c80f1d6122b9416ee37a28972 100644
--- a/Source/modules/webgl/WebGL2RenderingContextBase.idl
+++ b/Source/modules/webgl/WebGL2RenderingContextBase.idl
@@ -432,5 +432,8 @@ typedef long long      GLint64;
     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;