use idx for active camera over pointer

[PieterVdc]

split off from #4594

[Copilot]

Pull request overview

This PR replaces the PlayerInfo active-camera raw pointer with an index-based approach, introducing accessors and updating call sites to consistently fetch the active camera through a helper.

Changes:

• Replaced PlayerInfo::acamera with active_camera_idx and added get_player_active_camera() / set_player_active_camera().
• Updated gameplay, rendering, UI, networking, and audio code to use the new active-camera accessor instead of direct struct access.
• Adjusted view-mode switching and a few camera-dependent behaviors to tolerate missing/invalid camera pointers via checks.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/thing_traps.c	Fetches active camera via accessor before using rotation for trap shots.
src/thing_creature.c	Uses accessor for possession/control camera operations and zoom restore.
src/sounds.c	Uses accessor to locate local camera for footstep/ambient/receiver updates.
src/power_process.c	Uses accessor for lightning palette distance checks and lightning rendering.
src/player_utils.c	Keeps creature-info camera tracking aligned with active camera accessor.
src/player_instances.c	Updates many possession/query/zoom flows to use active camera accessor.
src/player_data.h	Replaces acamera field with active_camera_idx and declares new accessors.
src/player_data.c	Implements active camera getter/setter using the new index.
src/packets.c	Uses accessor for camera-driven packet processing and view restore.
src/net_checksums.c	Uses accessor to include camera position in desync snapshots/checksums.
src/main.cpp	Uses accessor for lightning proximity checks, save cleanup, and footsteps update.
src/local_camera.c	Uses accessor to decide whether first-person camera needs syncing.
src/light_data.c	Uses accessor to compute light render constraints from camera position.
src/gui_tooltips.c	Uses accessor for screen-to-map conversion when building gameplay tooltips.
src/frontmenu_ingame_map.c	Uses accessor for map overlays/rotation computations based on camera.
src/front_input.c	Uses accessor for bookmarks, map inputs, and screen-to-map conversion.
src/engine_render.c	Uses accessor for view-mode dependent rendering and local camera selection.
src/engine_redraw.c	Uses accessor when selecting active camera in view switching / redraw logic.
src/engine_camera.c	Uses accessor as the camera source for per-frame camera updates.
src/creature_control.c	Uses accessor to snapshot/restore view mode when starting creature control.

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