decklink: fixup internal time tracking over buffering pauses

Instead of relying on buffers after a state change to PLAYING to always start
from 0, track the amount of time we have spent outside playing but not changed
state to PAUSED.
3 jobs for decklink-buffering in 5 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #49594
manifest

00:00:26

 
  Build
passed #49595
build fedora x86_64

00:02:32

 
  Full Builds
passed #49596
build android arm64 api28

00:02:13