Mac: Fix inconsistent repeat of SDL_TEXTINPUT events on 10.7+

Due to the new "tap and hold" IME in Mountain Lion and above, we were getting
inconsistent repeat of SDL_TEXTINPUT events. Disabling that functionality (since
you can't see the popover anyway) solves this.

Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=2387