Fix #109822: Average Brush in Vertex Paint Mode is broken

Caused by 7a943428de

Looking at history, e.g. 575ade22d4 or prior, it seems the
"fallthrough" [related compiler warning was removed in 7a943428de] was
actually intended in the case of `VPAINT_TOOL_AVERAGE`.

So first, the average color is calculated and after that regular drawing
should happen with that color.

Now make this more clear by calling both `calculate_average_color` as
well as `vpaint_do_draw` before breaking.

Pull Request: https://projects.blender.org/blender/blender/pulls/109971

source/blender/editors/sculpt_paint/paint_vertex.cc

@@ -3695,6 +3695,7 @@ static void vpaint_paint_leaves(bContext *C,
   switch ((eBrushVertexPaintTool)brush->vertexpaint_tool) {
     case VPAINT_TOOL_AVERAGE:
       calculate_average_color(vpd, ob, me, brush, attribute, nodes);
+      vpaint_do_draw(C, sd, vp, vpd, ob, me, nodes, attribute);
       break;
     case VPAINT_TOOL_DRAW:
       vpaint_do_draw(C, sd, vp, vpd, ob, me, nodes, attribute);