Add generic signpost sprite for skins that don't have one

Sprite by MotorRoach. https://cdn.discordapp.com/attachments/321521470952177664/662520848808411136/srb20438.gif

Requires this: clearsign.pk3

changed the description

added 47 commits

• da226139...a6926e33 - 45 commits from branch next
• 6f5833f8 - Randomize skin during signpost roulette
• 5ce16950 - Merge branch 'next' into clear-sign-sprite

Compare with previous version

merged

mentioned in commit 1be64857