Fixes#7643
This commit address the issue with 3 minor fixes:
1. Initialize ghostty lib before app start, or global allocator will
be null.
2. `addSublayer` should be called on CALayer object, which is the
property 'layer' of UIView
3. According to apple's [document](https://developer.apple.com/documentation/metal/mtlstoragemode/managed?language=objc),
managed storage mode is not supported by iOS. So always use shared
mode.
FYI, another [fix](https://github.com/mitchellh/libxev/pull/204) in libxev
is also required to make iOS app work.
Zongyuan Li