Use absolute paths in flag image generation script
Due to a bug in Inkscape on macOS, the input and output files need to be specified with absolute paths.
I suggest making a small modification to the convert_png shell script to work around this bug on macOS.
Tested and seems to work as expected.
- macOS High Sierra 10.13.1
- Inkscape 0.92.2 5c3e80d, 2017-08-06 (Homebrew)