In gimp_operation_buffer_source_validate_process(), align the ROI
to the tile grid *before* intersecting it with the validate-
handler's dirty region.  This is necessary since, even though
subsequent operations will only read data within the ROI, the
entire tiles containing the ROI will be fetched, resulting in an
area potentially greater than the ROI.  We need to validate this
area in advance, or else it will be validated as part of the
subsequent operations, which can lead into the same deadlock we're
trying to prevent.