Fix #119396: Add toggle for texture paint mask icon

This PR adjusts the behavior of the Texture Paint mask icon to behave
similarly to the automasking icon in Sculpt mode. This behavior was
missed in the original task: #102585

Pull Request: https://projects.blender.org/blender/blender/pulls/119399

scripts/startup/bl_ui/space_view3d.py

@@ -995,7 +995,7 @@ class VIEW3D_HT_header(Header):
             row = layout.row()
             row.ui_units_x = 9
             row.popover(panel="VIEW3D_PT_slots_projectpaint", icon=icon)
-            row.popover(panel="VIEW3D_PT_mask", icon='MOD_MASK', text="")
+            row.popover(panel="VIEW3D_PT_mask", icon=VIEW3D_HT_header._texture_mask_icon(tool_settings.image_paint), text="")
         else:
             # Transform settings depending on tool header visibility
             VIEW3D_HT_header.draw_xform_template(layout, context)
@@ -1103,6 +1103,11 @@ class VIEW3D_HT_header(Header):
 
         return "CLIPUV_DEHLT" if automask_enabled else "CLIPUV_HLT"
 
+    @staticmethod
+    def _texture_mask_icon(ipaint):
+        mask_enabled = ipaint.use_stencil_layer or ipaint.use_cavity
+        return "CLIPUV_DEHLT" if mask_enabled else "CLIPUV_HLT"
+
 
 class VIEW3D_MT_editor_menus(Menu):
     bl_label = ""