screen: remove GetMainPosition()

Apply Y offsets manually in OnMouse().  This fixes a problem when
hiding the title bar.

src/screen.cxx

@@ -309,7 +309,16 @@ void
 ScreenManager::OnMouse(struct mpdclient &c, DelayedSeek &seek,
 		       Point p, mmask_t bstate)
 {
-	if (GetCurrentPage().OnMouse(c, p - GetMainPosition(), bstate))
+	if (options.show_title_bar) {
+		const int title_height = title_bar.GetHeight();
+		if (p.y < title_height) {
+			return;
+		}
+
+		p.y -= title_height;
+	}
+
+	if (GetCurrentPage().OnMouse(c, p, bstate))
 		return;
 
 	/* if button 2 was pressed switch screen */

src/screen.hxx

@@ -92,10 +92,6 @@ public:
 	void Init(struct mpdclient *c) noexcept;
 	void Exit() noexcept;
 
-	Point GetMainPosition() const noexcept {
-		return {0, (int)title_bar.GetHeight()};
-	}
-
 	const PageMeta &GetCurrentPageMeta() const noexcept {
 		return *current_page->first;
 	}