Add a concept for invocables with a return type

Because for some reason this is not part of the standard library.

src/common/utils.h

@@ -45,6 +45,17 @@ constexpr char product_name_override[] = "Get yabridge'd";
  */
 constexpr char vendor_name_override[] = "yabridge";
 
+/**
+ * The same as the `std::invocable` concept, but also specifying the result
+ * type.
+ */
+template <typename F, typename Result, typename... Args>
+concept invocable_returning = requires(F&& f, Result&& result, Args&&... args) {
+    {
+        std::invoke(std::forward<F>(f), std::forward<Args>(args)...)
+        } -> std::convertible_to<Result>;
+};
+
 // The cannonical overloading template for `std::visitor`, not sure why this
 // isn't part of the standard library
 template <class... Ts>

src/wine-host/utils.h

@@ -277,7 +277,7 @@ class MainContext {
      *   that will cause them to stall indefinitely in this situation, but who
      *   knows which other plugins exert similar behaviour.
      */
-    template <typename F, typename P>
+    template <std::invocable F, invocable_returning<bool> P>
     void async_handle_events(F handler, P predicate) {
         // Try to keep a steady framerate, but add in delays to let other events
         // get handled if the GUI message handling somehow takes very long.