Fix Metal layer detection in Qt 6.10 and newer

[niklaskorz]

As of Qt 6.10, the Metal layer is nested in an intermediary QContainerLayer:

qt/qtbase@0bdbf46

This came up while bumping vulkan-caps-viewer in nixpkgs, where Qt6 is at version 6.10.2.

Error message on launch without this fix:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[QContainerLayer naturalDrawableSizeMVK]: unrecognized selector sent to instance 0x600003c9cd80'

[niklaskorz]

Of course as-is, this patch would fail to launch on Qt <6.10, so let me know if you prefer a more flexible solution.

[SaschaWillems]

As the caps viewer is also packaged with the SDK, and that isn't done by me, changes like these need to be backwards compatible. So yes, in order to take this, it needs to properly work with the current Qt version.

[niklaskorz]

The function now first checks if the view's own layer is a CAMetalLayer already, and only if it's not and the contentLayer method is supported, it retrieves the nested content layer instead.