Previously tagging node positions dirty set the `PBVH_UpdateDrawBuffers`
flag which was then used by `node_draw_update_mask` and
`tag_all_attributes_dirty`, which, as hinted by the name, causes a refresh
of all the PBVH GPU vertex buffers, not just positions and normals.
Instead of using that flag, add a function to the draw cache to tag only
position-related data dirty. This should give a performance improvement
when there are also face sets, masks, and/or generic attributes being
extracted for drawing.
Part of $118145.
0c677e9494