Annoying spacing in mathed

[Jean-Marc Lasgouttes]

Now that I compile LyX with Qt5 by default on some of my computers, I 
see that, in linux, characters are annoyingly spaced (see attachments) 
with Qt5, but not Qt4.

I traced this back to QFontMetrics::rightBearing which returns different 
values. This value is used in mathed since 03a4b8c9328.

So, am I the only one seeing this (it is not present in windows)?

JMarc