Speed up Symbol drawing

When I call reloadDataInIndexRange on two points to change Symbols on tap, library hang main thread for about 500 ms. Here is backtrace from time profiler:

![zrzut ekranu 2014-03-17 o 15 16 51](https://f.cloud.github.com/assets/512353/2436775/c138f624-addf-11e3-85f4-58825773c901.png)

To fix it I use simple 2 lines of code, which redraw only Symbol rectangle.