The framenumber counter isn't necessarily equal to the output frame number

For example, our corporate template starts with `framenumber` 0 for the title page. I don't know if it's possible to figure this out automatically, but being able to pass a known offset as a package option would be good.

As a workaround, I inserted the following line before the definition of `pnote`, and replaced `theframenumber` with `pdfpcframenumber`:
```latex
\newcommand{\pdfpcframenumber}{\the\numexpr\value{framenumber}+1\relax}
```
That is completely unportable, of course, but could just as well be applied with an offset that defaults to 0.