Desktop Capture for macOS.

[Mac] feat: Support screen capture for macOS. (webrtc-sdk#24) (webrtc-sdk#36)
fix: Get thumbnails asynchronously. (webrtc-sdk#37)
fix: Use CVPixelBuffer to build DesktopCapture Frame, fix the crash caused by non-CVPixelBuffer frame in RTCVideoEncoderH264 that cannot be cropped. (webrtc-sdk#63)
Fix the crash when setting the fps of the virtual camera. (webrtc-sdk#62)

sdk/objc/native/src/objc_video_track_source.mm

@@ -37,7 +37,7 @@ - (void)capturer:(RTC_OBJC_TYPE(RTCVideoCapturer) *)capturer
 ObjCVideoTrackSource::ObjCVideoTrackSource() : ObjCVideoTrackSource(false) {}
 
 ObjCVideoTrackSource::ObjCVideoTrackSource(bool is_screencast)
-    : AdaptedVideoTrackSource(/* required resolution alignment */ 2),
+    : AdaptedVideoTrackSource(/* required resolution alignment */ is_screencast? 16 : 2),
       is_screencast_(is_screencast) {}
 
 ObjCVideoTrackSource::ObjCVideoTrackSource(RTC_OBJC_TYPE(RTCObjCVideoSourceAdapter) *adapter) : adapter_(adapter) {