shader: dual-source blending (Vulkan)

src/graphics/shader/shader.h

@@ -175,6 +175,7 @@ struct ShaderPixelInputInfo {
 	bool                                           ps_depth_export_enable       = false;
 	bool                                           ps_sample_mask_export_enable = false;
 	bool                                           ps_sample_shading            = false;
+	bool                                           dual_source_blending         = false;
 	bool                                           ps_early_z                   = false;
 	bool                                           ps_execute_on_noop           = false;
 	ShaderStageRuntime                             stage;