Workbench: Combine Xray Alpha with object/material alpha ...

... instead of overiding it (previous behavior).

In practice it's not really noticeable.
This means an object with alpha will never be more opaque when enabling
xray.

source/blender/draw/engines/workbench/workbench_data.c

@@ -24,6 +24,8 @@
 
 #include "DNA_userdef_types.h"
 
+#include "ED_view3d.h"
+
 #include "UI_resources.h"
 
 #include "GPU_batch.h"
@@ -55,6 +57,7 @@ void workbench_private_data_init(WORKBENCH_PrivateData *wpd)
     wpd->shading = v3d->shading;
     wpd->use_color_render_settings = false;
   }
+  wpd->shading.xray_alpha = XRAY_ALPHA(v3d);
 
   wpd->use_color_management = BKE_scene_check_color_management_enabled(scene);