Fixed potential scaling happening for maps used as tilesets

When determining the size of the tileset image, the map's layer offsets
were not taken into account. They were however taken into account when
determining the scale of the map. In effect, maps with layers at an
offset would get scaled down to fit on the too small image.

Closes mapeditor#2843

src/libtiled/minimaprenderer.cpp

@@ -76,7 +76,14 @@ MiniMapRenderer::~MiniMapRenderer()
 
 QSize MiniMapRenderer::mapSize() const
 {
-    return mRenderer->mapBoundingRect().size();
+    QRect mapBoundingRect = mRenderer->mapBoundingRect();
+    QSize mapSize = mapBoundingRect.size();
+
+    QMargins margins = mMap->computeLayerOffsetMargins();
+    mapSize.setWidth(mapSize.width() + margins.left() + margins.right());
+    mapSize.setHeight(mapSize.height() + margins.top() + margins.bottom());
+
+    return mapSize;
 }
 
 QImage MiniMapRenderer::render(QSize size, RenderFlags renderFlags) const
@@ -140,7 +147,7 @@ static void extendMapRect(QRect &mapBoundingRect, const MapRenderer &renderer)
     mapBoundingRect = rect.toAlignedRect();
 }
 
-void MiniMapRenderer::renderToImage(QImage& image, RenderFlags renderFlags) const
+void MiniMapRenderer::renderToImage(QImage &image, RenderFlags renderFlags) const
 {
     if (!mMap)
         return;