Cursor coordinates are viewport-relative (#17332)

The changeset is rather self-explanatory.
Some things in the rendering code are in
absolute and some things are in relative
coordinates. Cursor coordinates belong to
the latter. It's a bit confusing.

Closes #17310

## Validation Steps Performed
* Use the GDI text renderer
* Use cmd
* Press and hold Enter
* No more ghostly cursors ✅

src/renderer/base/renderer.cpp

@@ -432,7 +432,11 @@ bool Renderer::_CheckViewportAndScroll()
     // The cursor may have moved out of or into the viewport. Update the .inViewport property.
     {
         const auto view = ScreenToBufferLine(srNewViewport, _currentCursorOptions.lineRendition);
-        const auto coordCursor = _currentCursorOptions.coordCursor;
+        auto coordCursor = _currentCursorOptions.coordCursor;
+
+        // `coordCursor` was stored in viewport-relative while `view` is in absolute coordinates.
+        // --> Turn it back into the absolute coordinates with the help of the old viewport.
+        coordCursor.y += srOldViewport.top;
 
         // Note that we allow the X coordinate to be outside the left border by 1 position,
         // because the cursor could still be visible if the focused character is double width.