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>