Use NS::TransferPtr instead of NS::RetainPtr

[louwers]

I think this fixes a memory leak.

->alloc->init() sets the retain count to 1, then creating NS::RetainPtr increases it to two. The destructor of the shared pointer reduces the retain count to 1, resulting in a memory leak. So instead a NS::TransferPtr should be used. This is also what is done in other places of the codebase when calling ->alloc()->init()

/**
 * Create a SharedPtr by retaining an existing raw pointer.
 * Increases the reference count of the passed-in object.
 * If the passed-in object was in an AutoreleasePool, it will be removed from it.
 */
template <class _Class>
_NS_INLINE NS::SharedPtr<_Class> RetainPtr(_Class* pObject)
{
    NS::SharedPtr<_Class> ret;
    ret.m_pObject = pObject->retain();
    return ret;
}

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2254-compared-to-main.txt

[github-actions]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2254-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +20% +23.0Mi  +405% +24.2Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2254-compared-to-legacy.txt

[github-actions]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            +0.0015         +0.0016             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-2254-compared-to-main.txt