[ios, macos] Fix a memory leak when creating an EAGLContext in MGLMaSnapshotter

[fabian-guerra]

Fixes #11117 remaining leak.

[friedbunny]

Could you please briefly explain what the issue was and how this fixes it?

[fabian-guerra]

@friedbunny sure.

After profiling MGLMapSnapshotter I realized we had some memory leaks related to retaining self in the dispatch blocks. This was fixed by temporally retaining a weakSelf instance inside the blocks, releasing the underlaying _snapshotCallback and moving the attribution addition to it's own method. This PR addresses those issues #11133.

But there was still a leak that was keeping ~10MB per snapshot creation. The culprit for this leak was an instance of EAGLContext that was kept around. Here the context was created

mapbox-gl-native/platform/darwin/src/headless_backend_eagl.mm

Line 48 in 3a0c87b

EAGLContext* glContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];

and passed the ownership to

mapbox-gl-native/platform/darwin/src/headless_backend_eagl.mm

Line 11 in 3a0c87b

[reinterpret_cast<EAGLContext*>(glContext) retain];

Since this file headless_backend_eagl is marked as -fno-objc-arc HeadlessBackend::createContext() was not relinquishing its ownership thus making the leak.

This change makes EAGLBackendImpl the creator and owner of the context and the method HeadlessBackend::createContext() no longer holds a reference to it.

[fabian-guerra]

@ivovandongen @kkaefer looks like this change that Konstantin did "fixed" part of the problem in master and release-boba branches 335f04f#diff-71f34354e3b569a52e855d04f2bdeff3

[1ec5]

Since we didn’t mention #11133 in the v3.7.4 release notes, consider release-noting this fix, assuming it eliminates the remaining leakage.