Adds support for sizing options for the windowed mode.

I wanted to use pdfpc in a kiosk mode for an interactive presentation where I would also have videos playing on the screen.  To do this, I needed to force a particular size for the windowed mode so I could have the presentation on one side and my videos on the other.  This pull request implements a size option (in pixels) for the width and height of the window (forces windowed mode).