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/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{};