Blackmagic DeckLink plugin

The code specific to Windows: helps convert `BSTR` instances to
`std::string`s; provides a Windows COM-specific implementation of
`CreateDeckLinkDiscoveryInstance`; aliases CFUUIDGetUUIDBytes,
CFUUIDBytes, and IUnknownUUID (the Linux SDK does this, but for some
reason the Windows SDK does not).

Some changes were made to the stock DeckLink SDK: removed all references
to legacy API headers in DeckLinkAPI.idl; removed all instances of
`importlib("stdole2.tlb");`.

The code specific to Linux helps convert `const char *` instances to
`std::string`s.

The code specific to Mac OS X helps convert `CFString` instances to
`std::string`s.

This is a cross-platform Blackmagic device capture plugin that makes use
of the manufacturer provided DeckLink SDK.

ComPtr can be used for things like the DeckLink API, but ComQIPtr has
windows-specific compiler features, so can't be used outside of windows.

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In the same manner that PNG doesn't appear to work properly
with multiple threads, TIFF, JPEG2000 and WEBP also appears
to not render correctly (use of FFmpeg's ff_thread_* routines)
if decode is called before the automatic thread detection
has returned a suggested thread count for the decoder.

The reset method unnecessarily malloced a packet passed into
the packet buffer which does a dereferencing copy.

FFmpeg source updates: Clock chaining, frame dropping and friends

OBS_PROPERTIES_DEFER_UPDATE: Makes it so the properties are not updated
until editing of the properties is complete.

If this is omitted and you use an input that requires the network
you get a warning message about future versions not automatically
doing this for you.

This, if set, instructs the decoders to drop
frames if a specific timing window is not met.

This attaches clocks to packets and frames and defers
the start time until that particular frame is presented.
Any packets/frames in the future with the same clock
will reference that start time.

This fixes issues when there are multiple start times
in a large buffer (looped video/images/audio) and different
frames need different reference clocks to present correctly.

Enables clocks to wait if the main sync clock has not been started yet.  An example of this is a video stream (video/audio) being synced to the video clock.  If an audio frame gets produced before the video clock has started it will wait.

Add referencing counting to determine when to release a clock Due to no fixed ownership of clocks (packets, frames, decoders and  refresh thread all may own a clock at some point).

The bug was undetected because it accidentally fell into an error case that slept the correct amount of time.  pthread_cond_timedwait takes an absolute time in the future to wait until.  The value we were passing was always in the past so it was immediately failing with a TIMEDOUT error code.

This lets the decoder make decisions based on whether it is a hardware decoder or not.  Specifically, hardware decoders are more strict as to which frames can be dropped in an h264 stream.

This also replaces AVPacketList with ff_packet_list.

Added: US-West (San Francisco)

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