Use inverted defaultbg/fg for selection when bg/fg are the same

The background/foreground of selected text is currently set by setting ATTR_REVERSE, which flips its normal bg/fg. When the text being selected has the same bg and fg, it won’t be readable after selecting it, either.

The main use case is black-on-black text used to mark ‘spoilers’.

This patch allows that text to be read by selecting it, turning it into text with white bg and black fg (given default values for defaultbg/fg), just like most normal unformatted text when selected.