Support USB HID over AoAv2 mode for keyboard input

This provides a better input experience via
simulate physical keyboard, it converts
SDL_KeyboardEvent to proper HID events and send it
via HID over AoAv2.

This is a rewriting and bugfix of the origin code
from [@amosbird](https://github.com/amosbird).

Make sdl_keymod_to_hid_modifiers() more readable

Support MOD keys in HID mode

Enable Ctrl+V on HID mode

Support to send media events from hid_keyboard

Use existing --serial to replace --usb

Use explict option for input mode

Move HID keyboard setup code into functions

Send HID events in separated thread

Let libusb handle max package size

Fix HID keyboard report desc

README.md

@@ -207,6 +207,29 @@ scrcpy --crop 1224:1440:0:0   # 1224x1440 at offset (0,0)
 
 If `--max-size` is also specified, resizing is applied after cropping.
 
+#### USB HID over AOAv2
+
+Scrcpy can simulate a USB physical keyboard on Android to provide better input
+experience, you need to connect your device via USB, not wireless.
+
+However, due to some limitation of libusb and WinUSB driver, you cannot use HID
+over AOAv2 on Windows.
+
+Currently a USB serial number is needed to use HID over AOAv2.
+
+```bash
+scrcpy --serial XXXXXXXXXXXXXXXX # don't use HID
+scrcpy --serial XXXXXXXXXXXXXXXX --input-mode inject # don't use HID
+scrcpy --serial XXXXXXXXXXXXXXXX --input-mode hid # try HID and exit if failed
+```
+
+Serial number can be found by `adb get-serialno`.
+
+If you are a non-QWERTY keyboard user and using HID mode, please remember to set
+correct physical keyboard layout manually in Android settings, because scrcpy
+just forwards scancodes to Android device and Android system is responsible for
+converting scancodes to correct keycode on Android device (your system does this
+on your PC).
 
 #### Lock video orientation