shader_recompiler: Fix alpha-to-coverage fragment output interface

The Vulkan spec requires fragment shaders to declare an output covering
Location 0, Component 3 (alpha) when alpha-to-coverage is enabled. This change:

- Tracks alpha_to_coverage_enabled through RuntimeInfo from pipeline state
- Forces declaration of frag_color[0] with full RGBA when enabled
- Initializes alpha to 1.0 in shader epilogue if not explicitly written

Signed-off-by: Zephyron <zephyron@citron-emu.org>

src/shader_recompiler/backend/spirv/emit_spirv_special.cpp

@@ -1,4 +1,5 @@
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include "shader_recompiler/backend/spirv/emit_spirv.h"
@@ -61,6 +62,29 @@ Id ComparisonFunction(EmitContext& ctx, CompareFunction comparison, Id operand_1
     throw InvalidArgument("Comparison function {}", comparison);
 }
 
+void InitializeAlphaToCoverage(EmitContext& ctx) {
+    if (!ctx.runtime_info.alpha_to_coverage_enabled || !Sirit::ValidId(ctx.frag_color[0])) {
+        return;
+    }
+
+    // Load the current color value
+    const Id current_color{ctx.OpLoad(ctx.F32[4], ctx.frag_color[0])};
+
+    // Extract RGB components
+    const Id r{ctx.OpCompositeExtract(ctx.F32[1], current_color, 0u)};
+    const Id g{ctx.OpCompositeExtract(ctx.F32[1], current_color, 1u)};
+    const Id b{ctx.OpCompositeExtract(ctx.F32[1], current_color, 2u)};
+
+    // Set alpha to 1.0 for alpha-to-coverage
+    const Id alpha{ctx.Const(1.0f)};
+
+    // Reconstruct the color with alpha = 1.0
+    const Id new_color{ctx.OpCompositeConstruct(ctx.F32[4], r, g, b, alpha)};
+
+    // Store the updated color
+    ctx.OpStore(ctx.frag_color[0], new_color);
+}
+
 void AlphaTest(EmitContext& ctx) {
     if (!ctx.runtime_info.alpha_test_func) {
         return;
@@ -121,6 +145,7 @@ void EmitEpilogue(EmitContext& ctx) {
         ConvertDepthMode(ctx);
     }
     if (ctx.stage == Stage::Fragment) {
+        InitializeAlphaToCoverage(ctx);
         AlphaTest(ctx);
     }
 }

src/shader_recompiler/backend/spirv/spirv_emit_context.cpp

@@ -1,4 +1,5 @@
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <algorithm>
@@ -1664,7 +1665,8 @@ void EmitContext::DefineOutputs(const IR::Program& program) {
         break;
     case Stage::Fragment:
         for (u32 index = 0; index < 8; ++index) {
-            if (!info.stores_frag_color[index] && !profile.need_declared_frag_colors) {
+            if (!info.stores_frag_color[index] && !profile.need_declared_frag_colors &&
+                !(index == 0 && runtime_info.alpha_to_coverage_enabled)) {
                 continue;
             }
             frag_color[index] = DefineOutput(*this, F32[4], std::nullopt);

src/shader_recompiler/runtime_info.h

@@ -1,4 +1,5 @@
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #pragma once
@@ -93,6 +94,7 @@ struct RuntimeInfo {
     std::optional<float> fixed_state_point_size;
     std::optional<CompareFunction> alpha_test_func;
     float alpha_test_reference{};
+    bool alpha_to_coverage_enabled{};
 
     /// Static Y negate value
     bool y_negate{};

src/shader_recompiler/shader_info.h

@@ -1,4 +1,5 @@
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #pragma once
@@ -317,6 +318,7 @@ struct Info {
     bool uses_rescaling_uniform{};
     bool uses_cbuf_indirect{};
     bool uses_render_area{};
+    bool alpha_to_coverage_enabled{};
 
     IR::Type used_constant_buffer_types{};
     IR::Type used_storage_buffer_types{};