synchronize hightlights in non-/reflowing modes

and there is no need to OCR word if we can extract text from
original page in reflowing mode thanks to the rect maps between
normal page and reflowed page.

frontend/ui/geometry.lua

@@ -295,3 +295,13 @@ function Geom:midpoint(geom)
 	}
 end
 
+--[[
+return center point in this geom
+]]--
+function Geom:center()
+	return Geom:new{
+		x = self.x + self.w / 2,
+		y = self.y + self.h / 2,
+		w = 0, h = 0,
+	}
+end