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/scrcpy.h

@@ -33,6 +33,11 @@ enum sc_lock_video_orientation {
     SC_LOCK_VIDEO_ORIENTATION_3,
 };
 
+enum sc_keyboard_input_mode {
+    SC_KEYBOARD_INPUT_MODE_INJECT,
+    SC_KEYBOARD_INPUT_MODE_HID,
+};
+
 #define SC_MAX_SHORTCUT_MODS 8
 
 enum sc_shortcut_mod {
@@ -68,6 +73,7 @@ struct scrcpy_options {
     const char *v4l2_device;
     enum sc_log_level log_level;
     enum sc_record_format record_format;
+    enum sc_keyboard_input_mode keyboard_input_mode;
     struct sc_port_range port_range;
     struct sc_shortcut_mods shortcut_mods;
     uint16_t max_size;
@@ -112,6 +118,7 @@ struct scrcpy_options {
     .v4l2_device = NULL, \
     .log_level = SC_LOG_LEVEL_INFO, \
     .record_format = SC_RECORD_FORMAT_AUTO, \
+    .keyboard_input_mode = SC_KEYBOARD_INPUT_MODE_INJECT, \
     .port_range = { \
         .first = DEFAULT_LOCAL_PORT_RANGE_FIRST, \
         .last = DEFAULT_LOCAL_PORT_RANGE_LAST, \
@@ -151,6 +158,6 @@ struct scrcpy_options {
 }
 
 bool
-scrcpy(const struct scrcpy_options *options);
+scrcpy(struct scrcpy_options *options);
 
 #endif