Weight & Vertex Paint: always respect edit mode hiding on faces.

In some cases it is mandatory to be able to hide parts of the mesh
in order to paint certain areas. The Mask modifier doesn't work in
weight paint, and edit mode hiding requires using selection, which
is not always convenient.

This makes the weight and vertex paint modes always respect edit mode
hiding like sculpt mode. The change in behavior affects drawing and
building paint PBVH. Thus it affects brushes, but not menu operators
like Smooth or Normalize.

In addition, this makes the Alt-H shortcut available even without
any selection enabled, and implements Hide for vertex selection.

Differential Revision: https://developer.blender.org/D14163

source/blender/editors/include/ED_mesh.h

@@ -425,6 +425,9 @@ void paintvert_select_ungrouped(struct Object *ob, bool extend, bool flush_flags
 void paintvert_flush_flags(struct Object *ob);
 void paintvert_tag_select_update(struct bContext *C, struct Object *ob);
 
+void paintvert_hide(struct bContext *C, struct Object *ob, bool unselected);
+void paintvert_reveal(struct bContext *C, struct Object *ob, bool select);
+
 /* mirrtopo */
 typedef struct MirrTopoStore_t {
   intptr_t *index_lookup;