decklink: calculate the decklink output time from the internal clock

Fixes the time calculations when dealing with a slaved clock (as
will occur with more than one decklink video sink), when performing
flushing seeks causing stalls in the output timeline, pausing.

Tighten up the calculations by relying solely on the internal time
(from the internal clock) for determining when to schedule display
frames instead attempting to track pause lengths from the external
clock and converting to internal time.  This results in a much easier
offset calculation for choosing the output time and ensures that the
clock is always advancing when we need it to.

This is fixup to the 'monotonically increasing output timestamps' goal
in: bf849e9a
6 jobs for decklink-timing in 72 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #61394
gst indent

00:00:29

passed #61393
manifest

00:00:17

 
  Build
passed #61395
build fedora x86_64

00:01:36

 
  Test
passed #61396
check fedora

00:15:35

 
  Full Builds
passed #61397
build android arm64 api28

00:02:21

passed #61398
build cerbero fedora x86_64

00:55:11