Added MySQL support for 0004_auto__chg_field_rendition_focal_point_key

I ported the change from #827 to the 0004_auto__chg_field_rendition_focal_point_key south migration in the wagtailimages app. This allows me to execute runtests.py against a MySQL database. It still fails, but it at least doesn't crash.

I'm not certain that `db._get_connection().vendor` is the right way to do this, though. I vaguely recall finding an alternative to `db._get_connection()` some time ago, but searching through my old commits doesn't shed any light on what that alternative might have been.