Changed `View:get_content_offset()` to round resultant values

Avoids some issues that occur with fractional offsets, most noticable
on rectangles drawn on DocView jittering by 1-pixel when scrolling

data/core/view.lua

@@ -111,7 +111,9 @@ end
 
 
 function View:get_content_offset()
-  return self.position.x - self.scroll.x, self.position.y - self.scroll.y
+  local x = common.round(self.position.x - self.scroll.x)
+  local y = common.round(self.position.y - self.scroll.y)
+  return x, y
 end