Add helper function for ensuring a function is run in the GUI thread (#4091)

src/messages/Image.cpp

@@ -278,7 +278,10 @@ Image::~Image()
         return;
     }
 
-    // run destructor of Frames in gui thread
+    // Ensure the destructor for our frames is called in the GUI thread
+    // If the Image destructor is called outside of the GUI thread, move the
+    // ownership of the frames to the GUI thread, otherwise the frames will be
+    // destructed as part as we go out of scope
     if (!isGuiThread())
     {
         postToThread([frames = this->frames_.release()]() {

src/util/PostToThread.hpp

@@ -1,7 +1,8 @@
 #pragma once
 
-#include <QCoreApplication>
+#include "debug/AssertInGuiThread.hpp"
 
+#include <QCoreApplication>
 #include <QRunnable>
 #include <QThreadPool>
 
@@ -29,6 +30,19 @@ private:
     std::function<void()> action_;
 };
 
+template <typename F>
+static void runInGuiThread(F &&fun)
+{
+    if (isGuiThread())
+    {
+        fun();
+    }
+    else
+    {
+        postToThread(fun);
+    }
+}
+
 // Taken from
 // https://stackoverflow.com/questions/21646467/how-to-execute-a-functor-or-a-lambda-in-a-given-thread-in-qt-gcd-style
 // Qt 5/4 - preferred, has least allocations