Add support for glMapBufferRange. Use glMapBufferRange and glMapBufferSubData.

src/gpu/gl/GrGLAssembleInterface.cpp

Index: src/gpu/gl/GrGLAssembleInterface.cpp
diff --git a/src/gpu/gl/GrGLAssembleInterface.cpp b/src/gpu/gl/GrGLAssembleInterface.cpp
index aed11e539f74d70674c2830827522f3f50f570af..e4337259a22de23161fc1b6fd228e0ac1b97defd 100644
--- a/src/gpu/gl/GrGLAssembleInterface.cpp
+++ b/src/gpu/gl/GrGLAssembleInterface.cpp
@@ -173,6 +173,11 @@ const GrGLInterface* GrGLAssembleGLInterface(void* ctx, GrGLGetProc get) {
         GET_PROC(DeleteVertexArrays);
     }
 
+    if (glVer >= GR_GL_VER(3,0) || extensions.has("GL_ARB_map_buffer_range")) {
+        GET_PROC(MapBufferRange);
+        GET_PROC(FlushMappedBufferRange);
+    }
+
     // First look for GL3.0 FBO or GL_ARB_framebuffer_object (same since
     // GL_ARB_framebuffer_object doesn't use ARB suffix.)
     if (glVer >= GR_GL_VER(3,0) || extensions.has("GL_ARB_framebuffer_object")) {