obs-ffmpeg: Receive packets while frame queue is full

Previously, we always assumed we could push more frames before an encoded
packet would be available. Actually, we can get EAGAIN when the
frame queue is full and before any encoded packets have been completed.

Instead, attempt to read packets more often as the worst that can
happen is we get EAGAIN again.

plugins/obs-ffmpeg/obs-ffmpeg-vaapi.c

@@ -543,7 +543,7 @@ static bool vaapi_encode(void *data, struct encoder_frame *frame,
 	}
 
 	ret = avcodec_send_frame(enc->context, hwframe);
-	if (ret == 0)
+	if (ret == 0 || ret == AVERROR(EAGAIN))
 		ret = avcodec_receive_packet(enc->context, enc->packet);
 
 	got_packet = (ret == 0);