Opacity / Draw Ordering Refactor

[jinmin111]

Background

Alpha transparency is not applied correctly in the current Pygame surface layer-based implementation. The root cause is that blit ordering between surfaces and transparency compositing are not properly controlled.

Proposed Changes

• Preprocess draw calls and push them onto a stack instead of executing them directly from the engine
• Pop from the stack in z-order and render sequentially
• This allows control over which artist is visible first and which one appears on top
• Reduce dependency on surface layers and handle blitting at the artist level
• Define the role of layers as a logical z-order offset for the user

Implementation Approach

• Replace the per-layer _surface_dict with a single SRCALPHA render surface (+ a separate graph cache surface)
• Each frame: clear the render surface → blit graph cache onto it → draw all other artists in layer (z-order) order onto the same surface → blit render surface onto screen
• Layers are a z-order sorting key, not tied to separate surfaces
• Graph cache only redraws on camera move/zoom, everything else redraws each frame

Relevant Files (expected)

• pygame_engine.py
• render_manager.py

Closes

• Closes Transparent draws do not work #71
• Clean-up Graph Refactor #55 - _redraw_graph_artists() does not respect layer authority