Fix macOS build failure with older SDKs in QuartzWindow.mm

[chrisburr]

NSView.clipsToBounds is not declared in NSView.h in older macOS SDKs such as MacOSX11.0.sdk. This causes a build failure when compiling with these SDKs, as used by conda-forge:

QuartzWindow.mm:2618:10: error: property 'clipsToBounds' not found on object of type 'QuartzView *'
 2618 |     self.clipsToBounds = YES;
      |          ^
QuartzWindow.mm:2624:10: error: property 'clipsToBounds' not found on object of type 'QuartzView *'
 2624 |     self.clipsToBounds = self.subviews.count > 1;
      |          ^
2 errors generated.

Failing conda-forge build: conda-forge/root-feedstock#335

This PR replaces self.clipsToBounds = <value> with the equivalent self.layer.masksToBounds = <value>, guarded by self.wantsLayer = YES to ensure the view is layer-backed. layer.masksToBounds is the Core Animation equivalent and is available across all SDK versions.

Additionally, the overridden didAddSubview: and willRemoveSubview: methods now call their super implementations, which is standard practice when overriding these NSView methods.

[dpiparo]

Thanks. Starting the tests.

[github-actions]

Test Results

    22 files      22 suites   3d 5h 54m 12s ⏱️
 3 791 tests  3 791 ✅ 0 💤 0 ❌
75 369 runs  75 369 ✅ 0 💤 0 ❌

Results for commit eb3136f.

[vepadulano]

@chrisburr thank you for this PR!