drivers/glrend/cache: fix radii lights when inner == outer

First of all, it was meant to be 1e-6, not 1e6.
But regardless, that behaviour doesn't match what it did originally.

softrend/setup.c does this:

    if (lp->radius_inner != lp->radius_outer)
        alp->falloff = BR_DIV(alp->intensity, lp->radius_inner - lp->radius_outer);

i.e. no falloff if they're equal. Replicate this behaviour.

Author: vs49688

drivers/glrend/cache.c

@@ -124,7 +124,7 @@ static int casfd(br_gl_main_data_scene *scene, const state_light *in, size_t i,
             radii->radius_outer = in->radius_outer;
 
             if(radii->radius_inner == radii->radius_outer)
-                radii->radius_outer += 1e6f;
+                radii->radius_inner = 0;
 
             break;
     }