Add experimental Session Replay capturing on Mac

[tustanivsky]

This PR extends the opt-in Session Replay feature support to macOS allowing the SDK to continuously record the last several seconds of gameplay into a MP4 file and attach it to crash reports.

Example crash event with Session Replay attached (Mac).

Key Changes

• FMacSentrySubsystem now spawns the recorder in both backend modes (Cocoa and Native). The resulting .mp4 is attached via onLastRunStatusDetermined when using the Cocoa backend and via the in-process OnCrash hook when using the Native backend.

• FSentryBackBufferCapture now parameterizes its texture acquisition helpers over (format, flags, initial state), allowing the same FCachedTexture / FCachedTexturePool infrastructure to be shared across platforms.

• On macOS, the capture pipeline introduces an intermediate texture between the format-conversion draw pass and the CPU-readable pool slot, as Metal does not allow RenderTargetable and CPUReadback flags on the same texture.

Engine Dependencies

In addition to AVCodecsCore, Mac builds require the experimental VTCodecs plugin (Mac-only, ships with UE 5.2+). Both must be enabled in the project for USE_SENTRY_SESSION_REPLAY to be defined (guards session replay code).

Other Considerations

• Metal flag restrictions: VT's RHI bridge requires TexCreate_CPUReadback on the texture it receives, while Metal RHI does not allow TexCreate_CPUReadback and TexCreate_RenderTargetable to be used on the same texture. As a result, the macOS path performs one additional GPU copy from a singleton BGRA8 render-target intermediate into the CPU-readable texture from the pool.
• VT timestamp units: Encoder->SendFrame expects timestamps in microseconds on VT (whereas NVENC treats them as opaque values), and Packet.Timestamp is returned as mach_absolute_time host-clock ticks (whereas NVENC echoes the original value unchanged). The encoder thread now performs platform-specific timestamp conversion, scaling SendFrame timestamps to microseconds on macOS and converting packet timestamps via FPlatformTime::ToMilliseconds64.

Documentation

• docs(unreal): Session Replay capturing on Mac sentry-docs#18292

Related items

• Add experimental session replay capturing on Windows #1404

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[JoshuaMoelans]

lgtm, just a small fix on some bot-formatting + one follow-up question:
Q: could we have some tests for the session replay behaviour? I can imagine it's hard to capture actual session replays in CI, but maybe we can unit test certain parts of the flow?

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[tustanivsky]

Q: could we have some tests for the session replay behaviour? I can imagine it's hard to capture actual session replays in CI, but maybe we can unit test certain parts of the flow?

Testing replay capturing in CI doesn't seem particularly realistic (at least with GitHub-hosted runners) since it requires actual rendering to occur (non-null RHI) and the codec behavior is hardware-bound (NVENC requires an Nvidia GPU while VT runs only on macOS where we don't currently run checks at all).

The only component that's unit-testable in isolation is FSentryFMP4Writer as it's just byte-level muxing. That might be something we can cover in a follow-up PR though.