This implements pausing of outputs.  To accomplish this, raw audio/video
data is halted to the encoders or raw output.  Pausing is as precisely
timed as possible according to the timing of the obs_output_pause call,
and audio data will be spliced down to the exact audio sample in
accordance to that timing at the start/end marks.

Outputs that support this (outputs used for recording) can set the
OBS_OUTPUT_CAN_PAUSE capability flag.

(This commit also modifies the UI, obs-ffmpeg, and obs-output modules)

Fixes a long-time regression where the program would lock up if an
encode call fails.  Shuts down all outputs associated with the failing
encoder and displays an error message to the user.

Ideally, it would be best if a more detailed error could be displayed to
the user about the nature of the error, though the primary problem is
the encoder errors are typically not something the user would be able to
understand.  The current message is a bit of a generic error message;
improvement is welcome.

Another suggestion is to try to have the encoder restart seamlessly,
though it would take a significant amount of work to be able to make it
do something like that properly, and it sort of assumes that encoder
failures are sporadic, which may not necessarily be the case with some
hardware encoders on some systems.  It may be better just to use another
encoder in that case.  For now, seamless restart is ruled out.

Closes obsproject/obs-studio#1263