[yt-dev] Issue #1078: draw_id not working correctly in SlicePlot (yt_analysis/yt)

[dcollins4096]

New issue 1078: draw_id not working correctly in SlicePlot
https://bitbucket.org/yt_analysis/yt/issues/1078/draw_id-not-working-correctly-in-sliceplot

dcollins4096:

When running `SlicePlot` and `annotate_grids`, `draw_id` doesn't always display the correct grid ids.  

```
#!python

sl=yt.SlicePlot(ds,2,'x-velocity',center=[0.5,0.5,(n+0.5)/64.])
sl.annotate_grids(draw_ids=True)

```

One one data set, two consecutive slices have the same grid, 169, but they're clearly not the same grid:
http://imgur.com/a/mvCzW

A dataset to reproduce this can be find here:
http://use.yt/upload/56d3d0a3