In my previous implementation, I had the brightmapping of a texture
depend on the light level of the texture's line's front sector. This
looked quite realistic, as highlights were never fully brightmapped for
textures in very dark sectors, but on the other hand the brightmapping
would also oscillate for sectors with variable light levels and even
extinguish (!) when sectors go full dark. This was unacceptable. Now,
texture highlights are always brightmapped with full intensity.
With this change I consider my brightmaps implementation complete! \o/
What an unexpectedly athmospheric feature that I won't want to miss
anymore, thanks to all involved (especially @Nechaevsky for your
impressive support!). Fixes#246.