Fix bug #7518: Assert when selecting in RTL text

If we are in rtl text, we *need* to check whether the first character is a
space. We can't use the rtl variable for this because this is only
computed on the last row.

This bug was introduced in [0a137e31/lyxgit] to fix the computation for
freespacing paragraphs. We better can just check whether the paragraph is
freespacing or not.

(cherry picked from commit d7f68078cf)

src/TextMetrics.cpp

@@ -1230,8 +1230,9 @@ pos_type TextMetrics::getColumnNearX(pit_type const pit,
 
 	// if the first character is a separator, and we are in RTL
 	// text, this character will not be painted on screen
-	// and thus we should not count it and skip to the next.
-	if (rtl && par.isSeparator(bidi.vis2log(vc)))
+	// and thus we should not count it and skip to the next. Only
+	// in freespacing paragraphs, this first character is painted.
+	if (!par.isFreeSpacing() && par.isSeparator(bidi.vis2log(vc)))
 		++vc;
 
 	while (vc < end && tmpx <= x) {

status.20x

@@ -107,6 +107,8 @@ What's new
 - Fixed crash when modifying or pasting an equation label and there
   are references to that label in math (bug 8095).
 
+- Fixed assertion on selection of insets in RTL text (bug 7518).
+
 - Don't reset the selected format each time we click into a new paragraph
   in View->Source (bug 7997).