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/src/cli.c

@@ -279,6 +279,9 @@ scrcpy_print_usage(const char *arg0) {
         "    MOD+i\n"
         "        Enable/disable FPS counter (print frames/second in logs)\n"
         "\n"
+        "    Ctrl+click-and-move\n"
+        "        Pinch-to-zoom from the center of the screen\n"
+        "\n"
         "    Drag & drop APK file\n"
         "        Install APK from computer\n"
         "\n",