Fix band selection problems.

- Remove whitespace from the GridModel, which isn't set up to deal with
  it.

- Use absolute column bounds whenever translating the band rectangle
  into column indices.  This necessitates additional binary searching,
  but is necessary when columns aren't fully populated, which happens
  with the new grid layout.

BUG=26512327

Change-Id: I5016d5de8f00c366f575f0722739007ac1612923