Add support for USB HID keyboard over AOAv2

This provides a better input experience, by simulating a physical
keyboard. It converts SDL keyboard events to proper HID events, and send
them over AOAv2.

This is a rewriting and bugfix of the origin code from @amosbird:
<https://github.com/Genymobile/scrcpy/issues/279#issuecomment-453819354>

The feature is enabled the command line option -K or --hid-keyboard,
and is only available on Linux, over USB.

Refs <https://source.android.com/devices/accessories/aoa2#hid-support>
Refs <https://www.usb.org/sites/default/files/hid1_11.pdf>

PR #2632 <https://github.com/Genymobile/scrcpy/pull/2632>

Signed-off-by: Romain Vimont <rom@rom1v.com>

app/src/hid_keyboard.h

@@ -0,0 +1,42 @@
+#ifndef SC_HID_KEYBOARD_H
+#define SC_HID_KEYBOARD_H
+
+#include "common.h"
+
+#include <stdbool.h>
+
+#include "aoa_hid.h"
+#include "trait/key_processor.h"
+
+// See "SDL2/SDL_scancode.h".
+// Maybe SDL_Keycode is used by most people, but SDL_Scancode is taken from USB
+// HID protocol.
+// 0x65 is Application, typically AT-101 Keyboard ends here.
+#define SC_HID_KEYBOARD_KEYS 0x66
+
+/**
+ * HID keyboard events are sequence-based, every time keyboard state changes
+ * it sends an array of currently pressed keys, the host is responsible for
+ * compare events and determine which key becomes pressed and which key becomes
+ * released. In order to convert SDL_KeyboardEvent to HID events, we first use
+ * an array of keys to save each keys' state. And when a SDL_KeyboardEvent was
+ * emitted, we updated our state, and then we use a loop to generate HID
+ * events. The sequence of array elements is unimportant and when too much keys
+ * pressed at the same time (more than report count), we should generate
+ * phantom state. Don't forget that modifiers should be updated too, even for
+ * phantom state.
+ */
+struct sc_hid_keyboard {
+    struct sc_key_processor key_processor; // key processor trait
+
+    struct sc_aoa *aoa;
+    bool keys[SC_HID_KEYBOARD_KEYS];
+};
+
+bool
+sc_hid_keyboard_init(struct sc_hid_keyboard *kb, struct sc_aoa *aoa);
+
+void
+sc_hid_keyboard_destroy(struct sc_hid_keyboard *kb);
+
+#endif