Add png support

I started this PR few years ago :). This replace the screenshot output format ppm to png and add few options.

-add a new dependency to libpng
-save to png
-new shortcut to screenshot button: CTRL+P 

In not lightweigh mode:
-set the resolution (default 2048x2048)
-set the line width for the screenshot (default 1)
-possibility the save the transparency (default off)
-possibility to save the inverted transparency (default off)