When in greasepencil sculpt-/vertexpaint mode and using selection
masking, picking points wasnt working correctly.

So regular gpencil.select (without modifier keys) was not enabled for the
keymap. This only really makes sense for RCS (right click select) atm
(and not using RC fallback tools - which I dont think are present in
these modes anyways). With RCS, this can be supported and afaict, this
does not cause conflicts.