Task #1961450: Add Tibetan (bo) flag icon.

Downloaded SVG flag from Wikipedia[1], then ran it through `convert`[2]
and `pngcrush`[3].

[1] https://en.wikipedia.org/wiki/File:Flag_of_Tibet.svg
[2] convert -resize x12 -sigmoidal-contrast 10,50%
[3] pngcrush -brute -fix -force -reduce