Updated phpThumb to its current GitHub status, (1.7.12-pre).

The version of phpThumb that MODX currently ships, generates distorted thumbnails in some cases (e.g. when generating a horizontal thumbnail for a vertical image, with far=C). I figured out this can be fixed by updating phpThumb to v1.7.11 final.

Then I discovered that this library now lives in GitHub and the lines that concerned me (aspect ratio calculation) have been relocated once again after 1.7.11 (see JamesHeinrich/phpThumb@1603c41105ea88572cd9bc24a953df8415f76241), so I thought it might be better to update straight to their GitHub status instead.

If this change is too big however, I can make a new pull request which updates to 1.7.11 instead (or you can update the library yourself, that changeset is really small).