Initialize input manager dynamically

To simulate multitouch, we will need more fields in input_manager, which
will be initialized dynamically.

app/src/scrcpy.c

@@ -37,12 +37,7 @@ static struct decoder decoder;
 static struct recorder recorder;
 static struct controller controller;
 static struct file_handler file_handler;
-
-static struct input_manager input_manager = {
-    .controller = &controller,
-    .video_buffer = &video_buffer,
-    .screen = &screen,
-};
+static struct input_manager input_manager;
 
 // init SDL and set appropriate hints
 static bool
@@ -311,6 +306,8 @@ scrcpy(const struct scrcpy_options *options) {
         goto end;
     }
 
+    input_manager_init(&input_manager, &controller, &video_buffer, &screen);
+
     if (!server_connect_to(&server)) {
         goto end;
     }