[vulkan] Fixed dual-source blending to correctly map shader outputs. (#3637)

Fixes Skyward Sword HD eye gitch and a related MoltenVK crash due to the incorrect output mapping. Verified working on mac and android.

The test in vk_pipeline_cache.cpp is a bit ugly, but it didn't seem worth it to go lambda/macro just to make it look cleaner. Could change if necessary.

Co-authored-by: tarako <none@none.com>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3637
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>
Co-authored-by: tarako <r76036296@gmail.com>
Co-committed-by: tarako <r76036296@gmail.com>

src/shader_recompiler/backend/spirv/emit_spirv_special.cpp

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -91,6 +94,17 @@ void AlphaTest(EmitContext& ctx) {
 } // Anonymous namespace
 
 void EmitPrologue(EmitContext& ctx) {
+    if (ctx.stage == Stage::Fragment && ctx.runtime_info.dual_source_blend) {
+        // Initialize dual-source blending outputs - prevents MoltenVK crash.
+        const Id zero{ctx.Const(0.0f)};
+        const Id one{ctx.Const(1.0f)};
+        const Id default_color{ctx.ConstantComposite(ctx.F32[4], zero, zero, zero, one)};
+        for (u32 i = 0; i < 2; ++i) {
+            if (Sirit::ValidId(ctx.frag_color[i])) {
+                ctx.OpStore(ctx.frag_color[i], default_color);
+            }
+        }
+    }
     if (ctx.stage == Stage::VertexB) {
         const Id zero{ctx.Const(0.0f)};
         const Id one{ctx.Const(1.0f)};