Light flares (r_flares 2) fade state is not consistent across frames

[zturtleman]

Light flares (added by r_flares 2 or 3) connect fade state from the previous frame based on the light number added to the scene (see this line of tr_flares.c). This is not consistent as light are added or removed from the view. It can result in light flares being temporarily visible through walls.

This can probably be fixed using RTCW corona method of include an id (usually the entity number) to properly connect fade state across frames. Adding "visible" boolean to lights with result from collision trace like corona would help improve light flare performance by reducing framebuffer reads.

Issue found by Tobias Kuehnhammer.