video: unify screen-surface teardown into one helper

[ForkedInTime]

What

Extract the duplicated screen-surface / GL-backend teardown in bflib_video.c into a single lb_release_screen_surfaces() helper, and route both LbScreenSetup and LbScreenReset through it.

Why

The two functions each open-coded the same teardown, and they had drifted:

• LbScreenReset cleared both lbDrawSurface and lbScreenSurface.
• LbScreenSetup cleared only lbDrawSurface.

In GL-present mode those two globals alias the same owned surface, so after LbScreenSetup's teardown lbScreenSurface was left dangling at a just-freed surface — and stayed dangling on any early-return failure path (SDL_SetWindowDisplayMode/SDL_SetWindowFullscreen/window-create failure) before the new surface is assigned.

This is latent hardening: it was not the cause of the recent level-transition crash (that was a config/packaging issue, fixed separately), and repeated in-game resolution switches (Alt+R, the same teardown path) don't trip it in practice. But the asymmetry is a real dangling-pointer hazard and an easy trap for the next change.

Changes

• New static void lb_release_screen_surfaces(void) — frees the draw/second surface (and shuts down the GL backend if active), then clears lbHasSecondSurface, lbDrawSurface, lbScreenSurface, and lbScreenInitialised. Documents why both surface pointers must be cleared (GL alias) and why the CPU window-surface must not be freed.
• LbScreenSetup and LbScreenReset now call the helper instead of duplicating it — they can't diverge again.
• Removed the dead prevScreenSurf capture + empty if (prevScreenSurf != NULL) {} block in LbScreenSetup.
• Restructured the GL/second-surface free with a freed_draw_surface flag to drop the #ifndef _WIN32-straddling else (clears a clang-tidy misleading-indentation warning).

Verification

Behavior-preserving on the happy path; the only functional change is nulling lbScreenSurface in the setup teardown (strictly safer — it's reassigned before use, or left NULL instead of dangling on failure). Engine builds clean (make -f linux.mk, no new warnings on bflib_video.c).

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