Do not use . in image filenames

To prevent issues with `\includegraphics` for both pdflatex and tectonic,
see tectonic-typesetting/tectonic#187 .

Also, consolidate `$$opts{rasterize}`, `$$opts{pdf_plots}` and `$$opts{fmt}`
variables in `$$opts{img_fmt}`, which could support other formats in the
future (e.g. eps).