Revert "common/threadpool.c: silence compiler warning"

This reverts commit b328a555252611e9e7f3e7c7b6c978e4bb0fecb2.
Patch was too ugly.
stack_align: Change return value of x264_stack_align to intptr_t

Also change x264_slice_write which is the only indirect user
returning an integer.

This fixes the warning
common/threadpool.c: In function ‘x264_threadpool_thread’:
common/threadpool.c:69:20: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
on linux/x86_64 where sizeof(int) is 4 and sizeof(void *) is 8.

It also prevents problems in case the return value would ever be a
pointer that would be used.