WebGL 2: validate read buffer attachment when reading from FBO

Source/modules/webgl/WebGLFramebuffer.h

Index: Source/modules/webgl/WebGLFramebuffer.h
diff --git a/Source/modules/webgl/WebGLFramebuffer.h b/Source/modules/webgl/WebGLFramebuffer.h
index 9dd5ee170e36010458f9775f44c46448a7a2a26f..bdd3c7309e438dfc7a195cbc538ecfb0bdb76929 100644
--- a/Source/modules/webgl/WebGLFramebuffer.h
+++ b/Source/modules/webgl/WebGLFramebuffer.h
@@ -103,6 +103,10 @@ public:
 
     GLenum getDrawBuffer(GLenum);
 
+    void readBuffer(const GLenum colorBuffer) { m_readBuffer = colorBuffer; }
+
+    GLenum getReadBuffer() const { return m_readBuffer; }
+
     DECLARE_VIRTUAL_TRACE();
 
 protected:
@@ -134,6 +138,8 @@ private:
 
     Vector<GLenum> m_drawBuffers;
     Vector<GLenum> m_filteredDrawBuffers;
+
+    GLenum m_readBuffer;
 };
 
 } // namespace blink