Bug 24083

In a sequence like this:

create VBO
for (i=0;i<n;i++) {
    load new VBO data
    draw VBO
}
flush (really draw)

We're not flushing the previous drawing before the VBO contents are changed in the next iteration.  So whatever was last put in the VBO before the flush winds up getting drawn N times.  When we map the VBO to change its contents we need to note that a previous drawing command references the buffer and finish the drawing before allowing the VBO contents to be changed.

The attached program illustrates the problem.