TAB map content centred again; key bindings no longer pass on Ctrl/Alt

- megamap: the zoomed view (0x6cc950) is shared by the large map and the
  TAB overview map; its centre is now set per call from the drawing
  window's offset (hud::DrawOffsetX) instead of fixed to the render width
- input: drop the Ctrl/Alt pass-through; GetKeyState reports Alt as held
  after Alt+Tab, which switched all bindings back to the game's handling

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mods/hud.h

@@ -9,6 +9,10 @@ void OnDevice(void* device);
 // the current thread (0 outside): polled mouse positions are reduced by it.
 int CursorOffsetX();
 
+// Horizontal offset the current thread's UI drawing is shifted by (the
+// outermost hooked window's; 0 outside).
+int DrawOffsetX();
+
 // The mouse cursor object as the current thread should see it: inside a moved
 // window, a private copy with x shifted by CursorOffsetX().
 void* CursorFor(void* cursor);