Schema update erorr

[fgm]

I noticed what appears to be either a logic error in this code, or a Field API bug. Here is the issue, simple to reproduce:

• Install the latest version of the module
• create a "dim" field, 3-dimension, whichever widget, on a content type
• save the instance
• edit the instance, set it to 2-dimension
• save it: DBTNG error attempting to create a "depth" index although this
  dimension no longer exists
• field table no longer exists, but field definition remains in
  field_config[_instance] tables: manual cleaning needed in DB

This apparently happens because the hook_field_schema() implementation is
invoked twice during save, the first time with the old num_dimensions=3 value
and the second time with num_dimensions=2, but field API seems to get the
info from the first, pre-save, invocation, with the 3 columns in the schema.