Largest Canvas size allowed?

I am interested in advice concerning creating and operating on canvas sizes. (beyond 100,000 x 100,000 )
It appears there are some internal limits though I'm unsure if it is dask or datashader? Using 4096x4096 works, going to 8192 fails.
Thank you.

```
(Pdb) cvs=ds.Canvas(plot_width=8192,plot_height=8192)
(Pdb) agg=cvs.points(df, 'lon','lat', ds.count('epoch'))
*** MemoryError
```

I am attaching the stack trace.
[note.txt](https://github.com/bokeh/datashader/files/1148656/note.txt)