Remove getApp and getSettings calls from Message-Rendering (#4535)

* refactor: remove singletons from message rendering

* chore: add changelog entry

* Disable the `cppcoreguidelines-avoid-const-or-ref-data-members` clang-tidy check

* auto *app

* Selection is a struct, not a class

* Use ChannelView's `signalHolder_` instead of `channelConnections_`

* Remove `applySettings` step, instead just connect & set each setting individually

* rename & constify some context values

* Handle empty "last message color" setting value better (as it was
        originally in this pr before I removed that change :-)

* unrelated mini refactor cleanup

* let painSelection handle size_t instead of int

* Add some more comments to the MessageLayoutContext structs

---------

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/messages/layouts/MessageLayout.hpp

@@ -18,6 +18,7 @@ using MessagePtr = std::shared_ptr<const Message>;
 struct Selection;
 struct MessageLayoutContainer;
 class MessageLayoutElement;
+struct MessagePaintContext;
 
 enum class MessageElementFlag : int64_t;
 using MessageElementFlags = FlagsEnum<MessageElementFlag>;
@@ -49,9 +50,7 @@ public:
     bool layout(int width, float scale_, MessageElementFlags flags);
 
     // Painting
-    void paint(QPainter &painter, int width, int y, int messageIndex,
-               Selection &selection, bool isLastReadMessage,
-               bool isWindowFocused, bool isMentions);
+    void paint(const MessagePaintContext &ctx);
     void invalidateBuffer();
     void deleteBuffer();
     void deleteCache();
@@ -72,7 +71,7 @@ public:
 private:
     // methods
     void actuallyLayout(int width, MessageElementFlags flags);
-    void updateBuffer(QPixmap *pixmap, int messageIndex, Selection &selection);
+    void updateBuffer(QPixmap *buffer, const MessagePaintContext &ctx);
 
     // Create new buffer if required, returning the buffer
     QPixmap *ensureBuffer(QPainter &painter, int width);