Now that we have an option for "Correct Face Attributes", a very demanding
request is that this option also works with buttons.

This patch proposes this, but it has a problem:
Since it is not possible to find out when the callback of the button will
be last called, it is not possible to store a cache to correct face attributes.

Because of this, the cache is created and freed with each call which is highly
inefficient and loses a lot of precision.

Perhaps the most ideal is to create another customdata with the original values
from the bmesh coordinates. And every time a math customdata is requested, the
correction is made based on the original coordinates.

(But that would be a lot of work).