Draft: Tweak painting selection when line-height factor > 1.0

By painting the selection background like is done when the factor is 1.0 we get rid of a couple of painting issues; set it to > 1.0, then:

• select some text that contains (, ), { or }, make either edge of the selection right at one of those characters, the character is made bold, but also the heighlight matching brackets kicks in, so it's also highlighted e.g. yellow, the effect is very weird as with the selection background color being blue-ish, add yellow, and it's awful looking
• select a word, and use the go-to-next-match shortcut, one of the matches ends up being selected, but with the highlight similar words color still there, this seems to happen when the match is towards the top or bottom of the page, scrolling up/down fixes it (i.e. the highlight similar words color is cleared and only the selection background color remains)

Also by not making the selection background color span the whole line-height, I get selection that's more like Firefox when the line height > 1, i.e. with unpainted spaces between the selected lines.