Apple TV 4K 1st gen on tvOS 26 crashes to the home screen with a
"blocked by IOFence" gpuEvent at playback start (#949) and stutters
with a starved demuxer cache (#947). Defensive fixes in the shared
iOS/tvOS OpenGL path:
- Drain in-flight GPU work (glFinish) before destroying the
renderbuffer, so a pending presentRenderbuffer fence can't hold the
CAEAGLLayer IOSurface being torn down or reallocated
- Never leave the EAGLContext current on the main thread: framebuffer
create/destroy and the background glFinish now unbind on exit, and
setup no longer binds the context on main at all
- Disable retained backing on tvOS; it exists only for iOS PiP frame
capture and adds per-frame IOSurface fence pressure
- Report swaps to mpv after each present so display-vdrop (the tvOS
default video-sync) gets swap-timing feedback, matching macOS
Add verbose-gated diagnostics for remote triage: playback stats every
10s (cache fill/rate, dropped frames, hwdec, avsync) and a rate-limited
slow-frame warning in performRender.
Addresses #949 and #947
