Rather than passing a pointer to a waitgroup into task.Start(..)
functions, handle the waitgroup.Done() at the callsite.
This makes waitgroup handling local to its definition rather than it
being spread out over multiple files. Tasks now simply execute, and
the policy of waiting on them is handled by the caller.
* Generate screenshot images for markers
In some scenarios it might not be possible to use the preview video or
image of markers, i.e. when only static images are supported like in
Kodi. So generate a static screenshot as well.
* Make generating animated and static image optional for markers
* Use screenshot for markers when preview type is set to static image
* Implement parallel scanning and generation, and combined scanning/preview/sprite generation.
* Added UI component for preview/sprite generation during scan, and configurable number of parallel tasks.
* Add v050 changelog entry
Fixed annoyingly noisy transcoding progress log messages.
Fixed minor minor issue with directories than are named "blah.mp4" being detected as files to be scanned.
