This is the first step in what will eventually be keyframable time
remapping. The speed/duration dialog was a holdover from 0.1 and we can
probably do better here.
The workers run in separate threads meaning if any significant change is made
(e.g. parameters changing, or even closing the program), these workers may still
be mid-render. This is particularly problematic when closing since the nodes a
worker is rendering may be deleted mid-render. Render backends now have a
function that pauses the main thread (but starts a second event loop so the UI
isn't frozen) until the worker threads are all finished. This way, massive
changes can be made safely without race conditions.
Implemented the file dialog, save progress dialog, and separate thread to save
in (saving needs to be done in a separate thread so the main/GUI thread doesn't
get blocked).
An earlier commit implementing media in and out parameters was primarily for
this addition. A simple control dialog for the clip's speed presentation
reimplemented from the old codebase.
itsmattkc