Only base drag-select highlight on the legacy attribute.

The fg/bg values all have multiple representations, and some bits
have extra meanings, so inequality there doesn't mean they're not
the same colour.

This fixes an issue where the *second* drag select on a screen
would not highlight blue backgrounds correctly.  The background
colour gets the "pixels set" bit flown because of how
restorescreen() works, which caused this to "know" it's not blue
and use a blue background.

A (good) argument could be made that savescreen()/restorescreen()
shouldn't result in any changes, but since uifc32 doesn't directly
use colours that aren't represented by legacy attributes, there's
no need for it to get tricky here.