Update phpThumb to v1.7.13

I decided to compare core/model/phpthumb/phpthumb.class.php from the modx-2.3.0-pl.zip distribution against `develop` (and `master`) by looking through the [history](https://github.com/modxcms/revolution/commits/develop/core/model/phpthumb/phpthumb.class.php) for the MODX Revolution 2.3.0-pl release, and it looks like there were additional discrepancies:

**Compare these sequential commits:**
[phpthumb.class.php on 2014-03-10](https://github.com/modxcms/revolution/blob/e6cb4df4f90e80e0f4c8749678738a5e20266d16/core/model/phpthumb/phpthumb.class.php#L2248)
vs.
[phpthumb.class.php on 2014-07-15](https://github.com/modxcms/revolution/blob/f3b3bb5285353c837948e6e8b0f90106e20988fa/core/model/phpthumb/phpthumb.class.php#L2264)

...looks like in addition to the duplicate functions, this line was somehow changed?

Anyhow, I figured I would compare against phpThumb repo to validate, and saw the v1.7.13 release, hence this PR.

**NOTE:** Looks like the "weird character" � removed in 38eb388c718af8179eb15b42ccc14093fb812867 is actually part of [upstream](https://github.com/JamesHeinrich/phpThumb/blob/master/phpthumb.class.php#L1499), representing the "degree" character and should probably be left as is:

> // 'ra' may be part of this list, if not a multiple of 90°