libobs-metal: Wrap blitSwapChains in autoreleasepool to fix mach port leak

[alvin1995]

Description

Wraps the full frame cycle within blitSwapChains() in an autoreleasepool block, covering nextDrawable() → blit → commandBuffer.present(drawable) → commandBuffer.commit().

The blitSwapChains() method is driven by a CVDisplayLink callback thread, which has no implicit autorelease pool. CAMetalLayer.nextDrawable() returns autoreleased CAMetalDrawable objects, each backed by an IOSurface with an associated IOSurfaceSharedEventReference mach port. Without a pool, these autoreleased objects are never drained, leaking one IOSurface (and port) per frame.

Additionally, the drawable presentation order is corrected: drawables are now collected and presented via commandBuffer.present(drawable) after encoder.endEncoding() and before commandBuffer.commit(), matching the standard Metal ordering where presentation calls must occur after all encoding is complete but before the command buffer is committed.

Motivation and Context

This primarily manifests when using cross-process rendering via CAContext/CALayerHost (e.g., a source preview rendered in a separate process and composited into the main process). In that scenario swapChain.renderTarget is non-nil, so nextDrawable() is called per frame and the autoreleased drawables accumulate indefinitely, resulting in unbounded mach port growth.

The in-process projector path (OBS app's own preview) is masked because the on-screen compositor drives drawable recycling even without explicit draining — but the leak exists there too and can accumulate under sustained usage.

How Has This Been Tested?

• Environment: macOS, Apple Silicon, cross-process source preview via CAContext/CALayerHost.
• Method: Monitored mach ports via lsmp differential snapshots. Before the fix, IOSurfaceSharedEventReference ports grew linearly per frame with the preview window open. After the fix, port count stabilizes at a small upper bound (≈ pool size × swapchain count) and remains flat.
• Regression check: In-process preview continues to render correctly with no artifacts, no black frames, and no tearing.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing document.
• My code has been run through clang-format / swift-format.
• My code follows the project's style guidelines.
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation (N/A)