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/aoa_hid.h

@@ -0,0 +1,65 @@
+#ifndef SC_AOA_HID_H
+#define SC_AOA_HID_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#include <libusb-1.0/libusb.h>
+
+#include "scrcpy.h"
+#include "util/cbuf.h"
+#include "util/thread.h"
+
+struct sc_hid_event {
+    uint16_t accessory_id;
+    unsigned char *buffer;
+    uint16_t size;
+};
+
+// Takes ownership of buffer
+void
+sc_hid_event_init(struct sc_hid_event *hid_event, uint16_t accessory_id,
+                  unsigned char *buffer, uint16_t buffer_size);
+
+void
+sc_hid_event_destroy(struct sc_hid_event *hid_event);
+
+struct sc_hid_event_queue CBUF(struct sc_hid_event, 64);
+
+struct sc_aoa {
+    libusb_context *usb_context;
+    libusb_device *usb_device;
+    libusb_device_handle *usb_handle;
+    sc_thread thread;
+    sc_mutex mutex;
+    sc_cond event_cond;
+    bool stopped;
+    struct sc_hid_event_queue queue;
+};
+
+bool
+sc_aoa_init(struct sc_aoa *aoa, const char *serial);
+
+void
+sc_aoa_destroy(struct sc_aoa *aoa);
+
+bool
+sc_aoa_start(struct sc_aoa *aoa);
+
+void
+sc_aoa_stop(struct sc_aoa *aoa);
+
+void
+sc_aoa_join(struct sc_aoa *aoa);
+
+bool
+sc_aoa_setup_hid(struct sc_aoa *aoa, uint16_t accessory_id,
+              const unsigned char *report_desc, uint16_t report_desc_size);
+
+bool
+sc_aoa_unregister_hid(struct sc_aoa *aoa, uint16_t accessory_id);
+
+bool
+sc_aoa_push_hid_event(struct sc_aoa *aoa, const struct sc_hid_event *event);
+
+#endif