Partial frame capture API part 1

Define new optional struct in VideoFrame to signal that the frame is a
changed part of a whole picture and add a flag to signal that partial
update may be issued by the VideoFrame source.

Also, fix too strict assumptions in FrameBuffers PasteFrom methods.
Also, add ability to set a new buffer in video frame.


Bug: webrtc:10152
Change-Id: Ie0da418fd60bc7a34334329292e0b860ec388788
Reviewed-on: https://webrtc-review.googlesource.com/c/120405
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26489}

api/video/video_source_interface.h

@@ -42,6 +42,9 @@ struct RTC_EXPORT VideoSinkWants {
   absl::optional<int> target_pixel_count;
   // Tells the source the maximum framerate the sink wants.
   int max_framerate_fps = std::numeric_limits<int>::max();
+
+  // Tells the source that the sink supports partial frame updates.
+  bool partial_frames = false;
 };
 
 template <typename VideoFrameT>