Fix for compiler warnings on pointer casts

[pbleser]

Casting pointers as ints causes truncation and compiler warnings.
As those the pointers are just used in feh as means to pass any data, we just need to get rid of the compiler warnings.

The following patch uses a casting on intptr_t before casting to int to do so:
https://api.opensuse.org/public/source/X11:Utilities/feh/feh-fix_pointer_arithmetics.patch?rev=45ef01c1172dd974f39fed346d43073d

Furthermore, it uses %zu to print the intptr_t values, rather than %d (which isn't supported by ISO C90 but does the right thing on i586 and x86_64).

[derf]

Unable to reproduce with both gcc 4.6.2 and clang 3.0

Closing since I got no response to my question in #70.