ddnet did extend the tileset so it is no longer matching the 0.7 grass_main
renaming the file makes that clear and allows for further extensions of
the file without breaking teeworlds 0.7 compability.
Also it frees the filename grass_main_0.7.png to be used by the ddnet
0.7 client mode so it can display the correct mapres on 0.7 connections.
