[qt] Only use the MapRenderer as scheduler if there is no other

Optimization of when running on the Main Thread that has a RunLoop.

platform/qt/src/qmapboxgl_map_renderer.cpp

@@ -5,6 +5,7 @@
 QMapboxGLMapRenderer::QMapboxGLMapRenderer(qreal pixelRatio,
         mbgl::DefaultFileSource &fs, mbgl::ThreadPool &tp, QMapboxGLSettings::GLContextMode mode)
     : m_renderer(std::make_unique<mbgl::Renderer>(m_backend, pixelRatio, fs, tp, static_cast<mbgl::GLContextMode>(mode)))
+    , m_threadWithScheduler(Scheduler::GetCurrent() != nullptr)
 {
 }
 
@@ -48,19 +49,26 @@ void QMapboxGLMapRenderer::render()
     // The OpenGL implementation automatically enables the OpenGL context for us.
     mbgl::BackendScope scope(m_backend, mbgl::BackendScope::ScopeType::Implicit);
 
-    Scheduler::SetCurrent(this);
+    // If we don't have a Scheduler on this thread, which
+    // is usually the case for render threads, use this
+    // object as scheduler.
+    if (!m_threadWithScheduler) {
+        Scheduler::SetCurrent(this);
+    }
 
     m_renderer->render(*params);
 
-    std::queue<std::weak_ptr<mbgl::Mailbox>> taskQueue;
-    {
-        std::unique_lock<std::mutex> lock(m_taskQueueMutex);
-        std::swap(taskQueue, m_taskQueue);
-    }
+    if (!m_threadWithScheduler) {
+        std::queue<std::weak_ptr<mbgl::Mailbox>> taskQueue;
+        {
+            std::unique_lock<std::mutex> lock(m_taskQueueMutex);
+            std::swap(taskQueue, m_taskQueue);
+        }
 
-    while (!taskQueue.empty()) {
-        mbgl::Mailbox::maybeReceive(taskQueue.front());
-        taskQueue.pop();
+        while (!taskQueue.empty()) {
+            mbgl::Mailbox::maybeReceive(taskQueue.front());
+            taskQueue.pop();
+        }
     }
 }
 

platform/qt/src/qmapboxgl_map_renderer.hpp

@@ -48,4 +48,6 @@ private:
 
     std::mutex m_taskQueueMutex;
     std::queue<std::weak_ptr<mbgl::Mailbox>> m_taskQueue;
+
+    bool m_threadWithScheduler;
 };