Fix decals when there are world multiple scenes

Before decals where only projected in the first world scene of the frame and rendering multiple world scenes would replace decal draw surfaces from earlier scenes (causing decals to disappear in earlier scenes).

Now decals are projected in all world scenes and don't affect earlier scenes. All decal projectors added this frame when a scene is rendered will be used (including onces before the previous scene). So an explosion not visible in the first scene can be projected in later scenes. This seems to fit with how the trap_R_ProjectDecal system call is used.

Tested using a quad viewport patch I made: http://slexy.org/view/s2E96BAuzg