Fix crash doing a stroke in the weight paint

A regression since 2d2f4d9d28: the sculpt session is re-used by the
weight paint, so changes to semantic in one place requires same
changes in another.

Before the change the `original` was intiialized to false (due to the
clear-alloc), but after the change it was the accum with an inverted
meaning which was initialized to false.

Pull Request: https://projects.blender.org/blender/blender/pulls/107533

source/blender/editors/sculpt_paint/paint_vertex.cc

@@ -1657,6 +1657,8 @@ static void vwpaint_update_cache_invariants(
   copy_v3_v3(cache->view_normal, cache->true_view_normal);
   cache->bstrength = BKE_brush_alpha_get(scene, brush);
   cache->is_last_valid = false;
+
+  cache->accum = true;
 }
 
 /* Initialize the stroke cache variants from operator properties */