MixedModeRenderer non-72-dpi fixes & Pgf mixed rendering

I would like to add mixed mode rendering + test to `backend_pgf` for handling plots that are too complex for vectorized output (https://github.com/pwuertz/matplotlib-backend-pgf/issues/5).

A small fix was necessary since MixedModeRenderer assumes that the figure's dpi is always 72. I think I also fixed a bad indent at `stop_rasterizing` that was probably unnoticeable due to the fixed dpi assumption. According to a comment in the code @leejjoon might know if this is correct.