Enable dual source blending for Vulkan

src/gpu/vk/GrVkPipelineStateBuilder.cpp

Index: src/gpu/vk/GrVkPipelineStateBuilder.cpp
diff --git a/src/gpu/vk/GrVkPipelineStateBuilder.cpp b/src/gpu/vk/GrVkPipelineStateBuilder.cpp
index f4bb566de1588d5a70983c38b60b8628dfcf173d..a5ddd71d192b1eb8302111d2fba526138b9f423b 100644
--- a/src/gpu/vk/GrVkPipelineStateBuilder.cpp
+++ b/src/gpu/vk/GrVkPipelineStateBuilder.cpp
@@ -53,6 +53,10 @@ void GrVkPipelineStateBuilder::finalizeFragmentOutputColor(GrGLSLShaderVar& outp
     outputColor.setLayoutQualifier("location = 0");
 }
 
+void GrVkPipelineStateBuilder::finalizeFragmentSecondaryColor(GrGLSLShaderVar& outputColor) {
+    outputColor.setLayoutQualifier("location = 1");
+}
+
 VkShaderStageFlags visibility_to_vk_stage_flags(uint32_t visibility) {
     VkShaderStageFlags flags = 0;