Reduce cpu usage with x11grab

sunshine/platform/linux/x11grab.cpp

@@ -458,6 +458,7 @@ struct x11_attr_t : public display_t {
         std::this_thread::sleep_for((next_frame - now) / 3 * 2);
       }
       while(next_frame > now) {
+        std::this_thread::sleep_for(1ns);
         now = std::chrono::steady_clock::now();
       }
       next_frame = now + delay;
@@ -566,6 +567,7 @@ struct shm_attr_t : public x11_attr_t {
         std::this_thread::sleep_for((next_frame - now) / 3 * 2);
       }
       while(next_frame > now) {
+        std::this_thread::sleep_for(1ns);
         now = std::chrono::steady_clock::now();
       }
       next_frame = now + delay;