Fix scroller draining off-screen on low frame rates

The scroller recycled at most one line per frame while movement was
continuous (velocity * dt), so whenever a frame's displacement exceeded
one line height the text block drifted toward the boundary and the
screen drained with nothing replacing it. This surfaced on slower
hardware / 60Hz panels (|move| > lineHeight) but not on fast machines.

Recycle in a loop until every line that scrolled fully off-screen has a
replacement, for both scroll directions. addLineAt() now returns whether
a line was produced so the loop stops cleanly instead of losing lines.

src/scroller_page.h

@@ -79,7 +79,7 @@ private:
     void destroyResources();
     void rebuildWordTextures();
     void prefillLines();
-    void addLineAt(float baseY);
+    bool addLineAt(float baseY);
     void advanceStream();
     void switchAudioForCurrentStream();
     void updateAutoVelocity(float dt);