Add glReadBuffer to GrGLInterface.

Review URL: http://codereview.appspot.com/4841048/





git-svn-id: http://skia.googlecode.com/svn/trunk@2034 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/ios/GrGLDefaultInterface_iOS.cpp b/gpu/src/ios/GrGLDefaultInterface_iOS.cpp
index 8bc9fc5..7bdbcc7 100644
--- a/gpu/src/ios/GrGLDefaultInterface_iOS.cpp
+++ b/gpu/src/ios/GrGLDefaultInterface_iOS.cpp
@@ -76,6 +76,7 @@
         gDefaultInterface.fMatrixMode = glMatrixMode;
         gDefaultInterface.fPointSize = glPointSize;
         gDefaultInterface.fPixelStorei = glPixelStorei;
+        gDefaultInterface.fReadBuffer = NULL; 
         gDefaultInterface.fReadPixels = glReadPixels;
         gDefaultInterface.fScissor = glScissor;
         gDefaultInterface.fShadeModel = glShadeModel;