Add --rotation command-line option

In addition to Ctrl+Left and Ctrl+Right shortcuts, add a command-line
parameter to set the initial rotation.

app/scrcpy.1

@@ -110,6 +110,10 @@ Force recording format (either mp4 or mkv).
 .B \-\-render\-expired\-frames
 By default, to minimize latency, scrcpy always renders the last available decoded frame, and drops any previous ones. This flag forces to render all frames, at a cost of a possible increased latency.
 
+.TP
+.BI "\-\-rotation " value
+Set the initial display rotation. Possibles values are 0, 1, 2 and 3. Each increment adds a 90 degrees rotation counterclockwise.
+
 .TP
 .BI "\-s, \-\-serial " number
 The device serial number. Mandatory only if several devices are connected to adb.