Fix a crash with newest library

This is basically a replica of
4c0020385a
for Spectral.

src/imageprovider.h

@@ -2,23 +2,24 @@
 #define IMAGEPROVIDER_H
 #pragma once
 
-#include <jobs/mediathumbnailjob.h>
-#include <QThreadPool>
-#include <QtCore/QAtomicPointer>
-#include <QtCore/QReadWriteLock>
 #include <QtQuick/QQuickAsyncImageProvider>
+#include <QtCore/QAtomicPointer>
 
 namespace QMatrixClient {
 class Connection;
 }
 
 class ThumbnailResponse : public QQuickImageResponse {
+  Q_OBJECT
  public:
   ThumbnailResponse(QMatrixClient::Connection* c, QString mediaId,
                     const QSize& requestedSize);
   ~ThumbnailResponse() override = default;
 
+private slots:
   void startRequest();
+  void prepareResult();
+  void doCancel();
 
  private:
   QMatrixClient::Connection* c;
@@ -28,9 +29,8 @@ class ThumbnailResponse : public QQuickImageResponse {
 
   QImage image;
   QString errorStr;
-  mutable QReadWriteLock lock;
+  mutable QReadWriteLock lock; // Guards ONLY these two members above
 
-  void prepareResult();
   QQuickTextureFactory* textureFactory() const override;
   QString errorString() const override;
   void cancel() override;
@@ -41,7 +41,7 @@ class ImageProvider : public QObject, public QQuickAsyncImageProvider {
   Q_PROPERTY(QMatrixClient::Connection* connection READ connection WRITE
                  setConnection NOTIFY connectionChanged)
  public:
-  explicit ImageProvider() : QObject(), QQuickAsyncImageProvider() {}
+  explicit ImageProvider() = default;
 
   QQuickImageResponse* requestImageResponse(
       const QString& id, const QSize& requestedSize) override;
@@ -49,6 +49,7 @@ class ImageProvider : public QObject, public QQuickAsyncImageProvider {
   QMatrixClient::Connection* connection() { return m_connection; }
   void setConnection(QMatrixClient::Connection* connection) {
     m_connection.store(connection);
+    emit connectionChanged();
   }
 
  signals: