Append glyph index to ftgrid batch output

Appending the glyph index to the batch output file name is really helpful when setting up tests, so it would be great if there is an option to do that. i.e. if the glyph index is 45, ftgrid should output ftgrid45.png, instead of ftgrid.png

It's barely 2-3 line change, so I don't see any reason not to do it.

Also, unrelated, ftgrid should probably output an error if the system doesn't have libpng installed, instead of silently failing.