- eliminate an unexpected slow path in the fragment shader.

Apparently with checking uLightLevel the shader cannot discard the slow software lighting path entirely adding a significant amount of processing time.
Changed to check the actual lightmode value, which re-enables the fast path again.

wadsrc/static/shaders/glsl/main.fp

@@ -595,7 +595,7 @@ vec4 getLightColor(Material material, float fogdist, float fogfactor)
 {
 	vec4 color = vColor;
 
-	if (uLightLevel >= 0.0)
+	if ((uPalLightLevels >> 16) >= 5) // gl_lightmode >= 5 are software lighting modes.
 	{
 		float newlightlevel = 1.0 - R_DoomLightingEquation(uLightLevel);
 		color.rgb *= newlightlevel;