Callback procedure, informs of the insufficient number of free buffer pages available for use by the library. Procedure body has to be created elsewhere in the project (externally with respect to the library).
sub callback_agg_pre_buffrq(required_buff_pages as byte)
The number of additional buffer pages that the AGG library needs to operate. Your application must free up at least this number of buffer pages within callback_agg_pre_buffrq() or agg_start() will fail with the AGG_STATUS_INSUFFICIENT_BUFFER_SPACE code.
This procedure will be called only if there are not enough buffer pages available.