Inventory items clip into gameplay when closed; costumes draw over inventory background

[harrytyp]

Description

Two related inventory overlay rendering issues:

1. Inventory items clip into gameplay when the inventory is closed. Occasionally, inventory items (obj 117-274) render into the normal gameplay scene even though the inventory is not open. The pixel-diff culling heuristic in Step 2.5 lets them through in some frames.

2. Costumes are drawn on top of the inventory background. When the inventory IS open, actors/costumes render above the inventory background (obj 114). The inventory overlay should cover actors — the z-order is wrong.

Related code

• Step 2.5 culling thresholds (gfx.cpp, visiblePixels vs. threshold, FLOBJ 33% rule)
• Step 2.6 z-sort: sortKey = actorY - layer * 2000 — does not account for the full-screen inventory overlay

Related issues

• Inventory item HD textures never render — root cause + proposed fix (blast object position cache) #10 (inventory item HD rendering)
• Alpha issue: actor layering — objects rendered behind wrong actors (e.g. Larry's hook) #1 (actor layering)