Reallocate renderbuffer before checking if its dimensions have changed

This is a fix for #549.

Alternatively we might be able to avoid a reallocation in the case of a 180° rotation by looking at `mCinderView.layer.frame` first. Then again, `renderbufferStorage:fromDrawable:` might do that already behind the scenes. My personal feeling is that orientation changes don't happen very often, so it's okay to keep things simple.