Add pinch-to-zoom simulation

If Ctrl is hold when the left-click button is pressed, enable
pinch-to-zoom to scale and rotate relative to the center of the screen.

Fixes #24 <https://github.com/Genymobile/scrcpy/issues/24>

app/scrcpy.1

@@ -316,6 +316,10 @@ Inject computer clipboard text as a sequence of key events
 .B MOD+i
 Enable/disable FPS counter (print frames/second in logs)
 
+.TP
+.B Ctrl+click-and-move
+Pinch-to-zoom from the center of the screen
+
 .TP
 .B Drag & drop APK file
 Install APK from computer