Change volume shortcuts

I could not make Ctrl+'+' and Ctrl+'-' work for every keyboard on every
platform.

Instead, use Ctrl+UP and Ctrl+DOWN (like in VLC) to change the volume.

Fixes <https://github.com/Genymobile/scrcpy/issues/103>.

README.md

@@ -273,8 +273,8 @@ To run without installing:
  | click on `BACK`                        | `Ctrl`+`b` \| _Right-click²_  |
  | click on `APP_SWITCH`                  | `Ctrl`+`s`                    |
  | click on `MENU`                        | `Ctrl`+`m`                    |
- | click on `VOLUME_UP`                   | `Ctrl`+`+`                    |
- | click on `VOLUME_DOWN`                 | `Ctrl`+`-`                    |
+ | click on `VOLUME_UP`                   | `Ctrl`+`↑` _(up)_             |
+ | click on `VOLUME_DOWN`                 | `Ctrl`+`↓` _(down)_           |
  | click on `POWER`                       | `Ctrl`+`p`                    |
  | turn screen on                         | _Right-click²_                |
  | paste computer clipboard to device     | `Ctrl`+`v`                    |

app/src/inputmanager.c

@@ -29,11 +29,6 @@ static struct point get_mouse_point(struct screen *screen) {
     };
 }
 
-static SDL_bool is_ctrl_down(void) {
-    const Uint8 *state = SDL_GetKeyboardState(NULL);
-    return state[SDL_SCANCODE_LCTRL] || state[SDL_SCANCODE_RCTRL];
-}
-
 static void send_keycode(struct controller *controller, enum android_keycode keycode, const char *name) {
     // send DOWN event
     struct control_event control_event;
@@ -128,18 +123,6 @@ static void clipboard_paste(struct controller *controller) {
 
 void input_manager_process_text_input(struct input_manager *input_manager,
                                       const SDL_TextInputEvent *event) {
-    if (is_ctrl_down()) {
-        switch (event->text[0]) {
-            case '+':
-                action_volume_up(input_manager->controller);
-                break;
-            case '-':
-                action_volume_down(input_manager->controller);
-                break;
-        }
-        return;
-    }
-
     struct control_event control_event;
     control_event.type = CONTROL_EVENT_TYPE_TEXT;
     control_event.text_event.text = SDL_strdup(event->text);
@@ -189,6 +172,12 @@ void input_manager_process_key(struct input_manager *input_manager,
             case SDLK_p:
                 action_power(input_manager->controller);
                 return;
+            case SDLK_DOWN:
+                action_volume_down(input_manager->controller);
+                return;
+            case SDLK_UP:
+                action_volume_up(input_manager->controller);
+                return;
             case SDLK_v:
                 clipboard_paste(input_manager->controller);
                 return;

app/src/main.c

@@ -81,10 +81,10 @@ static void usage(const char *arg0) {
         "    Ctrl+m\n"
         "        click on MENU\n"
         "\n"
-        "    Ctrl+'+'\n"
+        "    Ctrl+Up\n"
         "        click on VOLUME_UP\n"
         "\n"
-        "    Ctrl+'-'\n"
+        "    Ctrl+Down\n"
         "        click on VOLUME_DOWN\n"
         "\n"
         "    Ctrl+p\n"