basetransform: `submit_input_buffer()` called with `is_discont=TRUE` all the time until output is produced
Currently priv->discont
is used to track whether input is discont or output is discont, and only reset to FALSE
once output is produced. That results in all inputs to be marked as discont until the first output is produced.
This is the case since 2015 when submit_input_buffer()
was introduced in a198803b.
CC @thaytan