Implement sub source for WebGL2's clearBuffer*().

third_party/WebKit/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.
 
 // https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7
 
 typedef long long GLint64;
 typedef unsigned long long GLuint64;
 
 [
@@ skipping 447 matching lines @@
     void vertexAttribIPointer(GLuint index, GLint size, GLenum type, GLsizei stride, GLintptr offset);
 
     /* Writing to the drawing buffer */
     void vertexAttribDivisor(GLuint index, GLuint divisor);
     void drawArraysInstanced(GLenum mode, GLint first, GLsizei count, GLsizei instanceCount);
     void drawElementsInstanced(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei instanceCount);
     void drawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, GLintptr offset);
 
     /* Multiple Render Targets */
     void drawBuffers(sequence<GLenum> buffers);
-    void clearBufferiv(GLenum buffer, GLint drawbuffer, Int32Array value);
-    void clearBufferiv(GLenum buffer, GLint drawbuffer, sequence<GLint> value);
-    void clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32Array value);
-    void clearBufferuiv(GLenum buffer, GLint drawbuffer, sequence<GLuint> value);
-    void clearBufferfv(GLenum buffer, GLint drawbuffer, Float32Array value);
-    void clearBufferfv(GLenum buffer, GLint drawbuffer, sequence<GLfloat> value);
+    void clearBufferiv(GLenum buffer, GLint drawbuffer, Int32Array value, optional GLuint srcOffset = 0);
+    void clearBufferiv(GLenum buffer, GLint drawbuffer, sequence<GLint> value, optional GLuint srcOffset = 0);
+    void clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32Array value, optional GLuint srcOffset = 0);
+    void clearBufferuiv(GLenum buffer, GLint drawbuffer, sequence<GLuint> value, optional GLuint srcOffset = 0);
+    void clearBufferfv(GLenum buffer, GLint drawbuffer, Float32Array value, optional GLuint srcOffset = 0);
+    void clearBufferfv(GLenum buffer, GLint drawbuffer, sequence<GLfloat> value, optional GLuint srcOffset = 0);
     void clearBufferfi(GLenum buffer, GLint drawbuffer, GLfloat depth, GLint stencil);
 
     /* Query Objects */
     WebGLQuery? createQuery();
     void deleteQuery(WebGLQuery? query);
     GLboolean isQuery(WebGLQuery? query);
     void beginQuery(GLenum target, WebGLQuery query);
     void endQuery(GLenum target);
     [CallWith=ScriptState] any getQuery(GLenum target, GLenum pname);
     [CallWith=ScriptState] any getQueryParameter(WebGLQuery query, GLenum pname);
@@ skipping 43 matching lines @@
     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, ArrayBufferView dstData, GLintptr offset);
     void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);
 };
 WebGL2RenderingContextBase implements WebGLRenderingContextBase;