Fix timing of paint-on captions.

Fix timing problem in write_char by updating
wb->data608->current_visible_start_ms when a character is added to an empty
use_buffer AND the use_buffer is the current visible buffer.

Added dbg_print to print start/end caption times.

Fixed bug where command, "RDC - Resume Direct Captioning" was being logged
as "RU1- Fake Roll up 1 rows.".