Implemented copyTexSubImage3D WebGL2 entry point in Blink

Source/modules/webgl/WebGL2RenderingContextBase.cpp

 // 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.
 
 #include "config.h"
 #include "modules/webgl/WebGL2RenderingContextBase.h"
 
 #include "bindings/modules/v8/WebGLAny.h"
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLImageElement.h"
@@ skipping 575 matching lines @@
     RefPtr<Image> image = videoFrameToImage(video);
     if (!image)
         return;
     texSubImage3DImpl(target, level, xoffset, yoffset, zoffset, format, type, image.get(), WebGLImageConversion::HtmlDomVideo, m_unpackFlipY, m_unpackPremultiplyAlpha);
 }
 
 void WebGL2RenderingContextBase::copyTexSubImage3D(GLenum target, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLint x, GLint y, GLsizei width, GLsizei height)
 {
     if (isContextLost())
         return;
-
-    notImplemented();
+    if (!validateCopyTexSubImage("copyTexSubImage3D", target, level, xoffset, yoffset, zoffset, x, y, width, height))
+        return;
+    WebGLFramebuffer* readFramebufferBinding = nullptr;
+    if (!validateReadBufferAndGetInfo("copyTexSubImage3D", readFramebufferBinding, nullptr, nullptr))
+        return;
+    clearIfComposited();
+    ScopedDrawingBufferBinder binder(drawingBuffer(), readFramebufferBinding);
+    webContext()->copyTexSubImage3D(target, level, xoffset, yoffset, zoffset, x, y, width, height);
 }
 
 void WebGL2RenderingContextBase::compressedTexImage3D(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLsizei depth, GLint border, DOMArrayBufferView* data)
 {
     if (isContextLost())
         return;
 
     WebGLTexture* tex = validateTextureBinding("compressedTexImage3D", target, true);
     if (!tex)
         return;
@@ skipping 1863 matching lines @@
 GLenum WebGL2RenderingContextBase::boundFramebufferColorFormat()
 {
     if (m_readFramebufferBinding && m_readFramebufferBinding->object())
         return m_readFramebufferBinding->colorBufferFormat();
     if (m_requestedAttributes.alpha())
         return GL_RGBA;
     return GL_RGB;
 }
 
 } // namespace blink