highlighter: Only recreate hi groups for visible buffers
highlighter: Only recreate hi groups for visible buffers

In a long editing session, there could happen to accumulate several
highlighting groups for buffers that might no longer be visible.

Therefore, only re-create the highlighting group for buffers that are
actually displayed in the current tabpage. If not, skip them.

references #1779

rchicoli/vim-airline,bling/vim-airline,yzhou61/vim-airline,chrisbra/vim-airline