fix: iOS - highlighting multiline mentions

Author: war-in

apple/MarkdownTextLayoutFragment.mm

@@ -82,14 +82,13 @@ - (void)drawMentions {
       CGFloat width = endLocation.x - startLocation.x;
       CGFloat x = lineBounds.origin.x + startLocation.x;
 
-      NSUInteger lineRelativeLocation = intersection.location - lineFragment.characterRange.location;
       UIFont *font = [lineFragment.attributedString attribute:NSFontAttributeName
-                                                      atIndex:lineRelativeLocation
+                                                      atIndex:intersection.location
                                                effectiveRange:NULL];
       CGFloat ascent = font.ascender;
       CGFloat descent = font.descender;
       CGFloat textHeight = ascent - descent;
-      CGFloat y = (startLocation.y - ascent) / 2;
+      CGFloat y = lineBounds.size.height * idx + (startLocation.y - ascent) / 2;
 
 
       CGRect backgroundRect = CGRectMake(x,