vk: Add checks for alphaToOne support

- This feature is very rarely used, as alphaToCoverage is commonly used as a replacement for blending, not in addition to it.

rpcs3/Emu/RSX/GL/GLGSRender.cpp

@@ -38,6 +38,7 @@ GLGSRender::GLGSRender() : GSRender()
 		m_vertex_cache = std::make_unique<gl::weak_vertex_cache>();
 
 	backend_config.supports_hw_a2c = false;
+	backend_config.supports_hw_a2one = false;
 	backend_config.supports_multidraw = true;
 }
 

rpcs3/Emu/RSX/RSXThread.h

@@ -469,6 +469,7 @@ namespace rsx
 		bool supports_multidraw;           // Draw call batching
 		bool supports_hw_a2c;              // Alpha to coverage
 		bool supports_hw_renormalization;  // Should be true on NV hardware which matches PS3 texture renormalization behaviour
+		bool supports_hw_a2one;            // Alpha to one
 	};
 
 	struct sampled_image_descriptor_base;

rpcs3/Emu/RSX/VK/VKGSRender.cpp

@@ -529,7 +529,11 @@ VKGSRender::VKGSRender() : GSRender()
 
 	// NOTE: We do not actually need multiple sample support for A2C to work
 	// This is here for visual consistency - will be removed when AA problems due to mipmaps are fixed
-	backend_config.supports_hw_a2c = (g_cfg.video.antialiasing_level != msaa_level::none);
+	if (g_cfg.video.antialiasing_level != msaa_level::none)
+	{
+		backend_config.supports_hw_a2c = VK_TRUE;
+		backend_config.supports_hw_a2one = m_device->get_alpha_to_one_support();
+	}
 
 	// NOTE: On NVIDIA cards going back decades (including the PS3) there is a slight normalization inaccuracy in compressed formats.
 	// Confirmed in BLES01916 (The Evil Within) which uses RGB565 for some virtual texturing data.
@@ -2587,12 +2591,14 @@ bool VKGSRender::load_program()
 	const auto rasterization_samples = u8((m_current_renderpass_key >> 16) & 0xF);
 	if (backend_config.supports_hw_a2c || rasterization_samples > 1)
 	{
+		const bool alpha_to_one_enable = rsx::method_registers.msaa_alpha_to_one_enabled() && backend_config.supports_hw_a2one;
+
 		properties.state.set_multisample_state(
 			rasterization_samples,
 			rsx::method_registers.msaa_sample_mask(),
 			rsx::method_registers.msaa_enabled(),
 			rsx::method_registers.msaa_alpha_to_coverage_enabled(),
-			rsx::method_registers.msaa_alpha_to_one_enabled());
+			alpha_to_one_enable);
 	}
 
 	properties.renderpass_key = m_current_renderpass_key;

rpcs3/Emu/RSX/VK/VKHelpers.h

@@ -809,6 +809,13 @@ private:
 				enabled_features.depthBounds = VK_FALSE;
 			}
 
+			if (!pgpu->features.alphaToOne && enabled_features.alphaToOne)
+			{
+				// AMD proprietary drivers do not expose alphaToOne support
+				LOG_ERROR(RSX, "Your GPU does not support alpha-to-one for multisampling. Graphics may be inaccurate when MSAA is enabled.");
+				enabled_features.alphaToOne = VK_FALSE;
+			}
+
 			VkDeviceCreateInfo device = {};
 			device.sType = VK_STRUCTURE_TYPE_DEVICE_CREATE_INFO;
 			device.pNext = nullptr;
@@ -931,6 +938,11 @@ private:
 			return pgpu->features.depthBounds != VK_FALSE;
 		}
 
+		bool get_alpha_to_one_support() const
+		{
+			return pgpu->features.alphaToOne != VK_FALSE;
+		}
+
 		mem_allocator_base* get_allocator() const
 		{
 			return m_allocator.get();