Add shadow and subtract styles to ACS.

There is a matching pull request for rheit/acs.
Restore shadow renderstyle.

It was accidentally lost in r3085.
Merge branch 'master' of https://github.com/Gaerzi/zdoom
Merge branch 'master' of https://github.com/rheit/zdoom
Merge branch 'master' of https://github.com/coelckers/gzdoom into osx