Fix Quartz clipboard image retrieval.

Fix Quartz clipboard image retrieval.

It does no good to iterate through a series of mime types to call a
function when the eventually-called function,
_gtk_quartz_get_selection_data_from_pasteboard() in this case, gives the
wrong answer and stops the iteration on all but one especially if that
one isn't first.

The one is "image/tiff" and the quartz pasteboard function will return
any image type Quartz knows about for it, so lose the iteration and use
only "image/tiff".