[x265] use std::swap() for readability

[Derek Buitenhuis]

On 7/9/2014 11:16 AM, Deepthi Nandakumar wrote:
> We spent a bunch of effort last year to remove STL dependencies, since they cause serious trouble between different compilers (even between different compiler versions).  This is especially since a lot of users will use x265 as a static library.

I tried to ask about this before to no avail: What compilers and how?

STL portability has not been an issue for a very long time.

- Derek