OpenGL2: Merge several cvars into r_pbr.

r_glossIsRoughness, r_specularIsMetallic, r_framebufferGamma, r_tonemapGamma, r_materialGamma, r_lightGamma

code/renderergl2/tr_scene.c

@@ -336,7 +336,7 @@ void RE_BeginScene(const refdef_t *fd)
 	else
 	{
 #if defined(USE_OVERBRIGHT)
-		float scale = pow(2, r_mapOverBrightBits->integer - tr.overbrightBits - 8);
+		float scale = (1 << (r_mapOverBrightBits->integer - tr.overbrightBits)) / 255.0f;;
 #else
 		float scale = (1 << r_mapOverBrightBits->integer) / 255.0f;
 #endif