Could not obtain dvipng version

I need to think about this. I'm pretty sure we can use

    > module-level texmanagers, since texmanagers dont keep any
    > state info (no args to __init__, no set_* functions
    > ...). On the other-hand, module-level texmanagers may be a
    > problem for someone who wants to do
    > rcParams['text.usetex']=True after they have imported
    > pylab.

This shouldn't pose any problem, because the functionality is only
called when usetex is True; they should be able to turn it on and off
at will.

I was more worried about changes in figure size and dpi. A quick
glance through the code suggests that these should be handled
properly, but I would test it before I changed it since as I mentioned
before I have this nagging feeling that there was a reason we put it
at the Renderer level.