/usr/share/tuxpaint/fonts/FreeSerif.ttf is in tuxpaint-data 1:0.9.22-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/tuxpaint/fonts/../../fonts/truetype/freefont/FreeSerif.ttf
/usr/share/tuxpaint/fonts/../../fonts/truetype/freefont/FreeSerif.ttf