This implements the SCULPT_vertex_is_boundary and SCULPT_vertex_has_unique_face_set functions for PBVH_GRIDS, which makes features such as automasking now work in multires. It also fixes some other face sets related features in multires, like face set boundary smoothing.

This uses the BKE_subdiv_ccg_coarse_mesh_adjacency_info_get function to get the vertex indicies in the base mesh from multires. This way the API functions can get topology or face set information directly from it. In the future, these vertex indices can be used to get any other information from the base mesh from multires, like seams, sharp edges, disconnected elements IDs...