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

Source/modules/webgl/WebGL2RenderingContextBase.h

Index: Source/modules/webgl/WebGL2RenderingContextBase.h
diff --git a/Source/modules/webgl/WebGL2RenderingContextBase.h b/Source/modules/webgl/WebGL2RenderingContextBase.h
index 19fa2d71ac579f0964b13dacd1b894a9edc1c7ab..2bdb7b10b7abb1e96c24d9ba08025a246c0e227e 100644
--- a/Source/modules/webgl/WebGL2RenderingContextBase.h
+++ b/Source/modules/webgl/WebGL2RenderingContextBase.h
@@ -158,6 +158,9 @@ public:
     GLboolean isVertexArray(WebGLVertexArrayObject*);
     void bindVertexArray(WebGLVertexArrayObject*);
 
+    /* Reading */
+    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, long long offset);
+
     /* WebGLRenderingContextBase overrides */
     void initializeNewContext() override;
     void bindFramebuffer(GLenum target, WebGLFramebuffer*) override;
@@ -165,6 +168,7 @@ public:
     ScriptValue getParameter(ScriptState*, GLenum pname) override;
     ScriptValue getTexParameter(ScriptState*, GLenum target, GLenum pname) override;
     ScriptValue getFramebufferAttachmentParameter(ScriptState*, GLenum target, GLenum attachment, GLenum pname) override;
+    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels) override;
     void restoreCurrentFramebuffer() override;
 
     EAGERLY_FINALIZE();