renderers: remove left over debug line in RE_ProjectDecal
renderers: use backEndData in RE_ProjectDecal

tr.refdef.decalProjectors always points to first element in backEndData->decalProjectors.
Using backEndData is more correct anyway because r_numDecalProjectors is used for access.
renderers: don't overwrite decal surfaces from earlier scenes this frame

Visible decals are added to tr.refdef.decals, which are then added to the scenes draw surfaces.
Modifying the decal surfaces in later scenes can cause decals to disappear from earlier scenes
(because they're replaced with different decals).
renderers: make decals work with multiple world scenes

Main view and portals/mirrors call R_CullDecalProjectors, as do multiple world scenes.
A decal not in main view frustum might be visible in portals/mirrors or another scene.
So don't kill decal projectors when culling or reordering them.

Add a new projectorNum variable to decalProjector_t and decal_t.
It's a global instance number so decals aren't projected on the same surface multiple times.

Add a new frameAdded variable to decal_t. It's used so that instant decals aren't reused the same frame.
It's needed so that instant decals aren't projected multiple times sometimes (when the decal was reused).

All decal projectors added this frame when a scene is rendered will be used (including onces before the last scene).
Merge pull request #95 from zturtleman/fix_decals_multiple_scenes

Fix decals when there are world multiple scenes