use single image per layer for overlay renders

include/ui/overlay.h

@@ -68,7 +68,7 @@ private:
 class Overlay
 {
 public:
-    Overlay() {
+    Overlay(QRect fromRect) : image(fromRect.size(), QImage::Format_ARGB32) {
         this->x = 0;
         this->y = 0;
         this->angle = 0;
@@ -77,10 +77,14 @@ public:
         this->hidden = false;
         this->opacity = 1.0;
         this->clippingRect = nullptr;
+        this->image.setOffset(fromRect.topLeft());
+        this->image.fill(QColor(0, 0, 0, 0));
+        this->valid = false;
     }
     ~Overlay() {
         this->clearItems();
     }
+    void invalidate() { this->valid = false; }
     bool getHidden();
     void setHidden(bool hidden);
     int getOpacity();
@@ -121,6 +125,8 @@ private:
     bool hidden;
     qreal opacity;
     QRectF *clippingRect;
+    QImage image;
+    bool valid;
 };
 
 #endif // OVERLAY_H

src/ui/graphicsview.cpp

@@ -74,7 +74,7 @@ void MapView::clearOverlayMap() {
 Overlay * MapView::getOverlay(int layer) {
     Overlay * overlay = this->overlayMap.value(layer, nullptr);
     if (!overlay) {
-        overlay = new Overlay();
+        overlay = new Overlay(this->sceneRect().toRect());
         this->overlayMap.insert(layer, overlay);
     }
     return overlay;

src/ui/overlay.cpp

@@ -25,6 +25,18 @@ void Overlay::renderItems(QPainter *painter) {
 
     painter->save();
 
+    // don't waste time if there are no updated to be made
+    if (!valid) {
+        QPainter overlayPainter(&this->image);
+        overlayPainter.setClipping(false);
+        QTransform t = overlayPainter.transform();
+        t.translate(-this->image.offset().x(), -this->image.offset().y());
+        overlayPainter.setTransform(t);
+        for (auto item : this->items) {
+            item->render(&overlayPainter);
+        }
+    }
+
     if (this->clippingRect) {
         painter->setClipping(true);
         painter->setClipRect(*this->clippingRect);
@@ -37,10 +49,11 @@ void Overlay::renderItems(QPainter *painter) {
     painter->setTransform(transform);
 
     painter->setOpacity(this->opacity);
-    for (auto item : this->items)
-        item->render(painter);
+    painter->drawImage(this->image.offset(), this->image);
 
     painter->restore();
+
+    valid = true;
 }
 
 void Overlay::clearItems() {
@@ -48,6 +61,7 @@ void Overlay::clearItems() {
         delete item;
     }
     this->items.clear();
+    this->image.fill(QColor(0, 0, 0, 0));
 }
 
 QList<OverlayItem*> Overlay::getItems() {