with-nth: Do not trim trailing whitespaces with background colors

Example:
  echo -en '  \e[48;5;232mhello\e[48;5;147m  ' | fzf --ansi --with-nth 1

src/core.go

@@ -126,7 +126,17 @@ func Run(opts *Options) (int, error) {
 				return false
 			}
 			item.text, item.colors = ansiProcessor(stringBytes(transformed))
-			item.text.TrimTrailingWhitespaces()
+
+			// We should not trim trailing whitespaces with background colors
+			var maxColorOffset int32
+			if item.colors != nil {
+				for _, ansi := range *item.colors {
+					if ansi.color.bg >= 0 {
+						maxColorOffset = util.Max32(maxColorOffset, ansi.offset[1])
+					}
+				}
+			}
+			item.text.TrimTrailingWhitespaces(int(maxColorOffset))
 			item.text.Index = itemIndex
 			item.origText = &data
 			itemIndex++

src/util/chars.go

@@ -184,9 +184,10 @@ func (chars *Chars) TrailingWhitespaces() int {
 	return whitespaces
 }
 
-func (chars *Chars) TrimTrailingWhitespaces() {
+func (chars *Chars) TrimTrailingWhitespaces(maxIndex int) {
 	whitespaces := chars.TrailingWhitespaces()
-	chars.slice = chars.slice[0 : len(chars.slice)-whitespaces]
+	end := len(chars.slice) - whitespaces
+	chars.slice = chars.slice[0:Max(end, maxIndex)]
 }
 
 func (chars *Chars) TrimSuffix(runes []rune) {