Add option to select video codec

Introduce the selection mechanism. Alternative codecs will be added in
further commits.

PR #3713 <https://github.com/Genymobile/scrcpy/pull/3713>

app/scrcpy.1

@@ -25,6 +25,10 @@ Encode the video at the given bit\-rate, expressed in bits/s. Unit suffixes are
 
 Default is 8000000.
 
+.TP
+.BI "\-\-codec " name
+Select a video codec (h264).
+
 .TP
 .BI "\-\-codec\-options " key\fR[:\fItype\fR]=\fIvalue\fR[,...]
 Set a list of comma-separated key:type=value options for the device encoder.