Add support for glMapBufferRange. Use glMapBufferRange and glMapBufferSubData.

src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp

Index: src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp
diff --git a/src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp b/src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp
index 6af047159a7ed4890bf6eefc61561fe7b648f205..d61afbd675d218a0402c909d080fea3e6749a4f2 100644
--- a/src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp
+++ b/src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp
@@ -132,6 +132,11 @@ const GrGLInterface* GrGLCreateNativeInterface() {
     functions->fUnmapBuffer = glUnmapBufferOES;
 #endif
 
+#if GL_EXT_map_buffer_range || GL_ES_VERSION_3_0
+    functions->fMapBufferRange = glMapBufferRange;
+    functions->fFlushMappedBufferRange = glFlushMappedBufferRange;
+#endif
+
 #if GL_APPLE_framebuffer_multisample
     functions->fRenderbufferStorageMultisample = glRenderbufferStorageMultisampleAPPLE;
     functions->fResolveMultisampleFramebuffer = glResolveMultisampleFramebufferAPPLE;