Sculpt: Line gestures and Mask Line tool

This adds support for line gesture to SculptGestureContext and
implements a Mask Line tool, which affects everything to the right of a plane
defined by the straightline gesture.

For this to work, a new WM_gesture_straightline_oneshot_modal is needed
which only runs exec when the gesture is over.

Added as experimental as it does not have icon.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D8722

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1258,6 +1258,21 @@ class _defs_sculpt:
             draw_settings=draw_settings,
         )
 
+    @ToolDef.from_fn
+    def mask_line():
+        def draw_settings(_context, layout, tool):
+            props = tool.operator_properties("paint.mask_line_gesture")
+            layout.prop(props, "use_front_faces_only", expand=False)
+
+        return dict(
+            idname="builtin.line_mask",
+            label="Line Mask",
+            icon="ops.sculpt.line_mask",
+            widget=None,
+            keymap=(),
+            draw_settings=draw_settings,
+        )
+
     @ToolDef.from_fn
     def face_set_box():
         def draw_settings(_context, layout, tool):
@@ -1273,6 +1288,7 @@ class _defs_sculpt:
             draw_settings=draw_settings,
         )
 
+
     @ToolDef.from_fn
     def face_set_lasso():
         def draw_settings(_context, layout, tool):
@@ -1308,7 +1324,6 @@ class _defs_sculpt:
             keymap=(),
         )
 
-
     @ToolDef.from_fn
     def mesh_filter():
         def draw_settings(_context, layout, tool):
@@ -2652,6 +2667,13 @@ class VIEW3D_PT_tools_active(ToolSelectPanelHelper, Panel):
                 _defs_sculpt.mask_lasso,
             ),
             _defs_sculpt.hide_border,
+            lambda context: (
+                (_defs_sculpt.mask_line,)
+                if context is None or (
+                        context.preferences.view.show_developer_ui and
+                        context.preferences.experimental.use_tools_missing_icons)
+                else ()
+            ),
             lambda context: (
                 (_defs_sculpt.face_set_box,)
                 if context is None or (