gpui: Fix native object accumulation in headless Metal rendering (#64099)

# Objective

Heavy headless rendering benchmarks accumulated native temporary objects
across frames because they could run for a long time without an
autorelease pool draining.

This PR adds a scoped autorelease pool around each headless rendering
call so those temporary objects are released promptly. GPU submission
remains asynchronous, and image readback retains its existing completion
wait.

Release Notes:

- N/A

Cargo.lock

@@ -7578,6 +7578,7 @@ dependencies = [
  "log",
  "metal",
  "objc",
+ "objc2 0.6.3",
  "parking_lot",
 ]