Update to ZDoom r4224:

    Instead of setting the default skybox in every sector without an explicit skybox set, set it once in the level struct and then use that for sectors with a NULL skybox. This fixes zpack's E2M3 so that when it removes its sector stacks, you will get the default skybox in their place, since stacked sectors and skyboxes use the same pointers in a sector.
    Added Xaser's reactiontime for ACS and DECORATE expression patch.

git-svn-id: http://mancubus.net/svn/hosted/gzdoom/trunk@1553 b0f79afe-0144-0410-b225-9a4edf0717df
- Fixed wrong casing of NULL.

git-svn-id: http://mancubus.net/svn/hosted/gzdoom/trunk@1554 b0f79afe-0144-0410-b225-9a4edf0717df