Fix Android SIGSEGV: don't free CGImage textures against a destroyed GL context

[tanarie]

Type of change: bug fix

Motivation

Fixes the ongoing Android crash:
https://app.bugsnag.com/flowkey/mobile-app/errors/69644a5e4ef4ae6af7fd0f05

SIGSEGV in renderer_GL_common.inl:3923, inside GPU_FreeImage. Android only.
Last 30 days: 108 events, 106 users; 198 events / 192 users all time. First seen
2026-01-12, still happening on 2.113.2. It correlates with the app going to the
background and coming back.

Root cause

On Android, backgrounding tears down the UIScreen and its GL context
(UIScreen.main = nil → UIScreen.deinit → GPU_Quit()); resuming builds a
new UIScreen/context. A CGImage created under the old context can
then be freed after the new context exists, and CGImage.deinit's guard only
checked UIScreen.main != nil — which passes, because a (new) screen exists. So
GPU_FreeImage ran against an image whose context_target is freed memory →
crash. The guard asked "does a screen exist?", not "is the context that owns
this image still alive?".

Why it reliably lands after the resume: deinit frees via Task { @MainActor },
and Android pauses our render loop while backgrounded, so jobs queued during
teardown only drain on the first frame after resume — the stack trace shows the
free inside nativeProcessEventsAndRender.

Why sdl-gpu's own guard doesn't help: it compares
image->renderer == GPU_GetCurrentRenderer(), but the renderer is SDL_malloc'd
and SDL_free'd (renderer_GLES_2.c:32/:62), so the GPU_Init after
GPU_Quit usually gets the same address back and the comparison passes
spuriously.

Fix

Tag each CGImage with the UIScreen.contextGeneration it was created under,
and only call GPU_FreeImage when that generation is still the current one —
in both deinit and reloadFromSourceData(). The counter is bumped whenever a
GL context is created or destroyed, so "generation matches" means exactly
"the context that owns this image is still live", without depending on
UIScreen.main being nil for the whole teardown→reinit window.

Skipping the free doesn't leak texture memory (that goes with the context), but
it does leak the small GPU_Image/GPU_IMAGE_DATA structs — FreeImage's two
SDL_free calls sit outside the GL branch. ~100 bytes per live image per
background cycle. Freeing them properly needs a registry of live CGImages
torn down before GPU_Quit(); follow-up, not a blocker.

Notes / testing

• Couldn't reproduce locally (POCO M3 / Adreno). Adreno keeps the GL context on
  background, so it rarely triggers; the crash mostly hits Samsung/Mali devices.
• Not verified locally — please review carefully, and confirm via the Bugsnag
  count dropping after release.
• Follow-up: bump submodules UIKit-SDL → NativePlayer → mobile-app once merged.

Please check if the PR fulfills these requirements

• Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• There are no dependencies on other PRs