Add ffmpeg capture to swaygrab (with limitations)

This needs to be multithreaded to have any sort of realistic expectation
of performance, due to issues with syncronous I/O.

swaygrab.1.txt

@@ -31,6 +31,13 @@ Options
 	Use the specified socket path. Otherwise, swaymsg will ask sway where the
 	socket is (which is the value of $SWAYSOCK, then of $I3SOCK).
 
+*-r, --rate* <rate>::
+	Specify a framerate (in frames per second). Used in combination with -c.
+	Default is 30. Must be an integer.
+
+*--raw*::
+	Instead of invoking ImageMagick or ffmpeg, dump raw rgba data to stdout.
+
 Examples
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