[agc] Reset transparent Chowdren effect-layer fills

[xnetcat]

Summary

• recognize Chowdren’s untextured transparent-black premultiplied fill as an effect-layer reset
• prevent Dreaming Sarah fog and vignette render targets from accumulating across frames
• provide SHARPEMU_DISABLE_TRANSPARENT_FILL_CLEAR=1 to restore the previous behavior

Scope

This is the minimal rendering portion of the earlier Dreaming Sarah investigation: 51 additions and one call-site change in AgcExports.cs. It deliberately excludes the unrelated guest-image synchronization and diagnostics work.

Validation

• $HOME/.dotnet/dotnet build SharpEmu.slnx --no-restore

[Copilot]

Pull request overview

This PR updates the AGC draw translation path to recognize a specific Chowdren “transparent black premultiplied fill” pattern as an effect-layer reset, preventing render-target accumulation across frames (notably for Dreaming Sarah fog/vignette). It also adds an opt-out environment variable to restore prior behavior.

Changes:

• Wraps the computed VulkanGuestRenderState with a new heuristic (ApplyTransparentPremultipliedFillClear) before creating TranslatedGuestDraw.
• Detects the “untextured + no vertex inputs + transparent RGBA + premultiplied One/OneMinusSrcAlpha” draw shape and disables blending to force an overwrite.
• Adds SHARPEMU_DISABLE_TRANSPARENT_FILL_CLEAR=1 to disable this behavior.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.