Viewer init was kind of too late since in many cases the sequence
would start caching and crash the app before the user could even read
the message. Moving to the startup makes it clearer from the beginning.
Improves minor issues with frame queue implemented in earlier commit. Also links
ViewerWindow with the queue and a separate timer to make it much smoother.
Rather than have to decode EXRs and display them in the span of one frame,
decode into memory ahead of time so that they're ready for upload. Has the
potential to speed up playback by about 5-6x.
itsmattkc