Remove direct memory access for security reasons

[Wormnest]

Copied from Bitbucket.
jacobb repo owner created an issue 2017-03-06

We are often directly accessing the memory of the image data and for most image types we don't always check if the current memory location is valid. This can be dangerous and cause buffer overflows which has serious security implications.

For that reason we should change our implementation to add access functions to the memory which functions check if the memory location is valid before returning the requested byte(s). No direct access should be allowed anymore.