This makes sure that libass already has all text buffers for the next frame
at the time when the next frame is processed. Without this text buffers
might be one frame late or not shown at all if they're only shown for a
single frame.
Use gst_element_klass_set_details_simple(), install properties
with static strings, create pads directly from the static pad
templates, directly put the segment into the instance struct,
use GST_MSECOND instead of numbers.
The PAR is a GstFraction and not a double, use gst_structure_get_fraction().
Simplify setcaps() functions.
