modify_gamma now use cache correctly

This exposed small bug in checkcache which checked just page no
which didn't get invalidated when closing page, so we now check if page
is also not nil

reader.lua

@@ -117,7 +117,7 @@ function freecache()
 end
 function checkcache(no)
 	for i = 1, #cache do
-		if cache[i].no == no then
+		if cache[i].no == no and cache[i].page ~= nil then
 			print("cache hit: slot="..i.." page="..no)
 			return i
 		end
@@ -216,13 +216,12 @@ function modify_gamma(offset)
 	if gamma == -1 then
 		gamma = 1
 	end
-	print("modify_gamma slot="..slot.." gamma="..gamma.." offset="..offset)
+	local no = cache[slot].no
+	print("modify_gamma "..no.." slot="..slot.." gamma="..gamma.." offset="..offset)
 	gamma = gamma + offset;
-	cache[slot].dc:setGamma( gamma );
 	optarg["G"] = gamma; -- for next page
-	cache[slot].page:draw(cache[slot].dc, cache[slot].bb, 0, 0)
-	fb:blitFullFrom(cache[slot].bb)
-	fb:refresh(0)
+	freecache()
+	goto(no)
 end
 
 function mainloop()