Use external file and callback to open file

[tsctrl]

Hi @Bodmer ,

thankyou for this great library.

few suggestion here:

1. move the file system support outside of the library, this help you to maintain this library and could prevent lots of other unrelated issue similar approach how you create rendering jpg function.
2. add configurable filesize limit check and return as JRESULT.
3. add malloc and free internally if that was not yet implemented.
4. keep everything else simple as is :)

thanks in advanced