ios: implement required mtkView:drawableSizeWillChange: selector

`MTKViewDelegate` declares two `@required` methods:
`mtkView:drawableSizeWillChange:` and `drawInMTKView:`. The
`QuadViewDlg` class only registered the latter, so any actual
drawable-size change (window resize, rotation under a non-locked
orientation set, split-view drag on iPad) crashed:

    *** Terminating app due to uncaught exception 'NSInvalidArgumentException',
    reason: '-[QuadViewDlg mtkView:drawableSizeWillChange:]:
    unrecognized selector sent to instance 0x...'
    ... [MTKView _resizeDrawable]
    ... [UIView setFrame:]
    ... [UIWindow _rotateWindowToOrientation:...]

Reproduced on the iPad Air 11-inch simulator on iOS 27 by rotating
the device. Did not reproduce on the iPhone 17 simulator there
because CHOMP's Info.plist locks orientation, so the drawable size
never actually changes.

Implementation is a stub: `draw_in_rect` already polls
`UIScreen.mainScreen.bounds` every frame and emits `Message::Resize`
on a delta, so a no-op satisfies the protocol without changing the
existing resize path.

Author: Benoît Rouleau

src/native/ios.rs

@@ -376,6 +376,19 @@ pub fn define_glk_or_mtk_view_dlg(superclass: &Class) -> *const Class {
         draw_in_rect(this, s, o, nil);
     }
 
+    // `MTKViewDelegate` requires both `drawInMTKView:` AND
+    // `mtkView:drawableSizeWillChange:`. MTKView invokes the
+    // size-change selector before the first `drawInMTKView:` after
+    // any drawable-size change (window resize, rotation under a
+    // non-locked orientation set, split-view drag on iPad). Without
+    // an implementation, `_resizeDrawable` raises
+    // `NSInvalidArgumentException` and crashes the process.
+    //
+    // The real resize handling lives in `draw_in_rect`, which polls
+    // `UIScreen.mainScreen.bounds` every frame and emits
+    // `Message::Resize` on a delta — so a stub is enough here.
+    extern "C" fn drawable_size_will_change(_: &Object, _: Sel, _: ObjcId, _: NSSize) {}
+
     unsafe {
         decl.add_method(
             sel!(glkView: drawInRect:),
@@ -386,6 +399,11 @@ pub fn define_glk_or_mtk_view_dlg(superclass: &Class) -> *const Class {
             sel!(drawInMTKView:),
             draw_in_rect2 as extern "C" fn(&Object, Sel, ObjcId),
         );
+
+        decl.add_method(
+            sel!(mtkView: drawableSizeWillChange:),
+            drawable_size_will_change as extern "C" fn(&Object, Sel, ObjcId, NSSize),
+        );
     }
 
     decl.add_ivar::<*mut c_void>("display_ptr");