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

Source/modules/webgl/WebGLRenderingContextBase.h

Index: Source/modules/webgl/WebGLRenderingContextBase.h
diff --git a/Source/modules/webgl/WebGLRenderingContextBase.h b/Source/modules/webgl/WebGLRenderingContextBase.h
index fb2d4636476f6c9d2c02aead671317d4d38229e3..f3c605055790cd29db2a430b5c57631416a1d46c 100644
--- a/Source/modules/webgl/WebGLRenderingContextBase.h
+++ b/Source/modules/webgl/WebGLRenderingContextBase.h
@@ -250,6 +250,7 @@ public:
     void pixelStorei(GLenum pname, GLint param);
     void polygonOffset(GLfloat factor, GLfloat units);
     void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels);
+    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, long long offset);
     void renderbufferStorage(GLenum target, GLenum internalformat, GLsizei width, GLsizei height);
     void sampleCoverage(GLfloat value, GLboolean invert);
     void scissor(GLint x, GLint y, GLsizei width, GLsizei height);
@@ -489,6 +490,8 @@ protected:
     // List of bound VBO's. Used to maintain info about sizes for ARRAY_BUFFER and stored values for ELEMENT_ARRAY_BUFFER
     PersistentWillBeMember<WebGLBuffer> m_boundArrayBuffer;
 
+    PersistentWillBeMember<WebGLBuffer> m_boundPixelPackBuffer;
+
     PersistentWillBeMember<WebGLVertexArrayObjectBase> m_defaultVertexArrayObject;
     PersistentWillBeMember<WebGLVertexArrayObjectBase> m_boundVertexArrayObject;
     void setBoundVertexArrayObject(WebGLVertexArrayObjectBase* arrayObject)
@@ -821,6 +824,10 @@ protected:
     // Generates INVALID_OPERATION and returns false if the combination is unsupported.
     bool validateReadPixelsFormatTypeCombination(GLenum format, GLenum type, GLenum readBufferInternalFormat, GLenum readBufferType);
 
+    // Helper function to check parameters of readPixels. Returns true if all parameters
+    // are valid. Otherwise, generates appropriate error and returns false.
+    bool validateReadPixelsFuncParameters(GLsizei width, GLsizei height, GLenum format, GLenum type, unsigned domArrayBufferLength, WebGLFramebuffer*& readFramebufferBinding);
+
     virtual GLint getMaxTextureLevelForTarget(GLenum target);
 
     // Helper function to check input level for functions {copy}Tex{Sub}Image.