drive map rendering from main thread

src/mbgl/map/map_context.cpp

@@ -269,7 +269,11 @@ void MapContext::update() {
             }
         }
 
-        view.invalidate([this] { render(); });
+        if (mayRender) {
+            render();
+        } else {
+            view.invalidate();
+        }
     }
 
     updated = static_cast<UpdateType>(Update::Nothing);
@@ -302,9 +306,16 @@ void MapContext::renderStill(StillImageCallback fn) {
     }
 
     callback = fn;
+    mayRender = true;
     triggerUpdate(Update::RenderStill);
 }
 
+void MapContext::renderSync() {
+    mayRender = true;
+    render();
+    mayRender = false;
+}
+
 void MapContext::render() {
     assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
 
@@ -326,11 +337,15 @@ void MapContext::render() {
     painter->setDebug(data.getDebug());
     painter->render(*style, transformState, data.getAnimationTime());
 
+    mayRender = false;
+
     if (data.mode == MapMode::Still) {
         callback(nullptr, view.readStillImage());
         callback = nullptr;
     }
 
+    view.swap();
+
     // Schedule another rerender when we definitely need a next frame.
     if (data.transform.needsTransition() || style->hasTransitions()) {
         triggerUpdate();
@@ -351,7 +366,7 @@ void MapContext::setSourceTileCacheSize(size_t size) {
         for (const auto &source : style->sources) {
             source->setCacheSize(sourceCacheSize);
         }
-        view.invalidate([this] { render(); });
+        view.invalidate();
     }
 }
 
@@ -361,7 +376,7 @@ void MapContext::onLowMemory() {
     for (const auto &source : style->sources) {
         source->onLowMemory();
     }
-    view.invalidate([this] { render(); });
+    view.invalidate();
 }
 
 void MapContext::onTileDataChanged() {