Add back in original PNG renderer

[saxbophone]

Function name: sxbp_render_figure_to_png() (same signature as sxbp_render_figure_to_null().

Already working code exists for this in the original project (v0.27.0).

This might be a good time for me to add an additional rendering option that handles a text comment (PNG output for example builds the file with a comment referring to the library).