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.cpp

@@ -431,6 +431,10 @@ int CursorOffsetX() {
     return g_cursorDx;
 }
 
+int DrawOffsetX() {
+    return g_offsetX;
+}
+
 void* CursorFor(void* cursor) {
     return g_cursorDx != 0 && cursor == g_cursorShadow.real ? g_cursorShadow.bytes : cursor;
 }