Added the Atreides grenadier

Was part of the 1.06 (christmas) patch http://web.archive.org/web/20000229040341/http://www.westwood.com/corporate/press/c_pr98_019.html for Dune 2000 http://dune2k.com/Duniverse/Games/Dune2000/Downloads/Patches However this can also be merged after the release.

The sprite is an old file from http://content.open-ra.org/index.php?p=detail&table=units&id=328 when the R8 → PNG → SHP(TD) conversion was done half-automatically. This is now broken because we export R8 into frames with different dimensions which SHP(TD) does not support. I also failed to rip the parts I need from 1.06 DATA.R8 with https://github.com/Mailaender/OpenRA/compare/file-position and a hex editor.