Only regard images as pixbufs if they have identity transformations

Only regard images as pixbufs if they have identity transformations

In order for a src/mask pair to be considered a pixbuf, they have to
have identical transformations, but we don't check for that. Since the
only fast paths we have for pixbufs require identity transformations,
it sufficies to check that both source and mask are
untransformed.

This is also the reason that this bug can't be triggered by any test
code - if the source and mask had different transformations, we would
consider them a pixbuf, but then wouldn't take the fast path because
at least one of the transformations would be different from the
identity.