I use show-paren-style in mixed mode, and would like to be able to use
different faces depending on whether the match is being displayed in
'expression style or 'parenthesis (I want a stronger contrast in
parenthesis than in expression). The attached patch splits the
show-paren-match face into two, show-paren-match-expression and
show-paren-match, using inheritance to make the split styling opt-in.

With this change, users can style the two faces separately if they wish.