perf: add signal to invalidate buffers without doing layout (#5123)

src/singletons/WindowManager.hpp

@@ -66,6 +66,10 @@ public:
     // This is called, for example, when the emote scale or timestamp format has
     // changed
     void forceLayoutChannelViews();
+
+    // Tell a channel (or all channels if channel is nullptr) to invalidate all paint buffers
+    void invalidateChannelViewBuffers(Channel *channel = nullptr);
+
     void repaintVisibleChatWidgets(Channel *channel = nullptr);
     void repaintGifEmotes();
 
@@ -124,6 +128,9 @@ public:
     // This signal fires whenever views rendering a channel, or all views if the
     // channel is a nullptr, need to redo their layout
     pajlada::Signals::Signal<Channel *> layoutRequested;
+    // This signal fires whenever views rendering a channel, or all views if the
+    // channel is a nullptr, need to invalidate their paint buffers
+    pajlada::Signals::Signal<Channel *> invalidateBuffersRequested;
 
     pajlada::Signals::NoArgSignal wordFlagsChanged;