Don't run lookahead on every line of frame-by-frame typesetting

This is an accuracy tradeoff - not knowing the next sub right away
can cause speed flicker but I haven't noticed real-world regressions

Author: po5

evafast.lua

@@ -208,9 +208,7 @@ local function adjust_speed()
 
             if sub_displayed then
                 target_speed = options.subs_speed_cap
-            end
-
-            if options.subs_lookahead then
+            elseif options.subs_lookahead then
                 if next_sub_at < current_time and next_sub_at ~= -2 then
                     next_sub_at = next_sub(current_time)
                 end